ABSTRACT VOLUME
THE 9th IGNEOUS AND METAMORPHIC STUDIES GROUP MEETING
15-18 January 2017
Glenburn Lodge, Muldersdrift
Organised by:
Marlina Elburg, Jérémie Lehmann, Trishya Owen-Smith & Sebastian Tappe
Department of Geology, University of Johannesburg
Sponsored by:
UNUSUAL FEATURES WITHIN THE STRATIGRAPHY OF THE CRITICAL ZONE SOUTH OF
MOKOPANE
Acheampong, K.O1., Kinnaird, J.A1., Nex, P.A.M.1
1
School of Geosciences, University of the Witwatersrand, Wits 2050, South Africa;
[email protected]
The mafic rocks of the Bushveld Complex differ to the north and south of the Ysterberg-Planknek Fault, part
of the array that is the Thabazimbi-Murchison Lineament. To the north, there is sporadic Lower Zone,
succeeded by a PGE-Cu-Ni-bearing Platreef, Main Zone and Upper Zone successions. To the south is Lower
Zone, the Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member and Main Zone. This investigation on
farm of Grasvally aims to provide a better understanding of the mafic rocks south of Mokopane, in order to
provide an improved understanding of the relationship between the geology and mineralisation of this region.
The farm Grasvally is situated approximately 20 km south of Mokopane. The geology of the Grasvally area
comprises mainly mafic to ultramafic Bushveld rocks of Lower Zone to Main Zone affinity which were
emplaced above sediments of the Pretoria Group. The GNPA member was originally divided into two major
sub-units by Hulbert (1983) but more recently it has been divided into Lower Mafic Unit (LMF), the Lower
Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO) (de Klerk, 2005). The GNPA member
may be the equivalent of the Platreef to the north of the Ysterberg-Planknek Fault. The ThabazimbiMurchison Lineament may have acted as a barrier to the flow of magma causing the GNPA member to be
compartmentalised and follow a different evolution from Critical Zone magma in the eastern or western
limbs or the Platreef to the north. Previous work carried out at Grasvally was minimal, apart from detailed
work by Hulbert (1983). Smith et al. (2011) provided an in depth account of the petrography and mineralogy
of silicates, oxides and base metal sulphides in the GNPA member to the north of this study area.
In this project, 20 borehole cores have been studied and logged and the stratigraphy compared and tentatively
correlated with the work of de Klerk (2005). These cores were drilled in 2004 by Samancor and were made
available for this study by Sylvania Platinum Ltd. Core logging indicated that the GNPA member overlies
the Magaliesberg quartzites of the Pretoria Group and the Lower Zone to the east and the west of the farm
respectively. Previous work (Smith et al., 2014) suggests PGE mineralisation is associated with sulphides
and is not restricted to one lithological unit within the GNPA member. Within the LGN blebs (<1 cm) of
fine-grained cumulus magnetite within norite have not been documented previously. Additionally, below this
unit, within the LMF are disseminated to well-developed layers of chromite, as well as isolated pods of
chromitite within norite and gabbronorite which are not easily correlated with the known stratigraphy. The
boreholes also intersect a pyroxenite unit within the LMF that has visual similarities, mineralisation, the
same hanging and footwall, as the UG 2 chromitite layer. This pyroxenite comprising orthopyroxene,
interstitial plagioclase and clinopyroxene hosts disseminated to well-developed chromitite layer with
associated disseminated to blebby pyrrhotite and chalcopyrite. Two well-developed chromitites ~60-90 cm
are intersected within the Lower Zone (LZ). The chromitites, disseminated chromite, pyrrhotite, chalcopyrite
and minor pentlandite mineralisation are hosted within the harzburgites and pyroxenites of the LZ.
de Klerk, L. 2005. Bushveld Stratigraphy on Rooipoort, Potgietersrus Limb [abs.], 2nd Platreef Workshop,
Mokopane, South Africa, October, Abstracts
Hulbert, L. J., 1983, A petrological investigation of the Rustenburg layered suite and associated
mineralization south of Potgietersrus.
Smith, J., Holwell, D., and McDonald, I., 2011, The mineralogy and petrology of platinum-group elementbearing sulphide mineralisation within the Grasvally Norite–Pyroxenite–Anorthosite (GNPA)
member, south of Mokopane, northern Bushveld Complex, South Africa: Applied Earth Science, v.
120, no. 4, p. 158-174.
Smith, J., Holwell, D. A., and McDonald, I., 2014, Precious and base metal geochemistry and mineralogy of
the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South
Africa: implications for a multistage emplacement: Mineralium Deposita, v. 49, no. 6, p. 667-692.
2 EUDIALYTE OR NO EUDIALYTE - CONTRASTING TRENDS OF AGPAITIC
CRYSTALLIZATION IN NEPHELINE SYENITE OF THE PILANESBERG COMPLEX, SOUTH
AFRICA
Tom Andersen1,2, Marlina A. Elburg1 & Muriel Erambert2
1
Department of Geology, University of Johannesburg, South Africa; [email protected]
2
Department of Geosciences, Oslo University, Norway, [email protected],
[email protected]
Peralkaline nepheline syenites can roughly be divided in two families: the miaskitic, in which Zr and Ti are
hosted in zircon, titanite and ilmenite, and the agpaitic, in which these common HFSE minerals are not
stable. Common to most agpaitic rocks is the presence of members of the eudialyte group (complex Na-CaMn-Fe-Zr-Cl silicates), but other minerals such as Na-Ca-Ti-Zr-F disilicates and aenigmatite
(Na2Fe5TiSi6O20) are also diagnostic. There is general consensus that the main driving force behind the
miaskitic-to-agpaitic transition is an increase in the alkali content of the magma but other factors such as
halogen and water content are also important (e.g. Andersen et al., 2010).
Two types of peralkaline nepheline syenite from the Pilanesberg complex that show different
magmatic crystallization histories have been investigated in this study: white foyaite from the outer ring
segment (the Matooster type of Retief, 1963) and members of the green foyaite suite from the southern rim
of the complex. Although the major and trace element composition of the rocks are different (Elburg et al.,
2017), both groups have crystallized magmatic mineral assmblages with alkali feldspars, nepheline, sodalite,
fluorite and sodic pyroxene and amphibole.. The white foyaite magma started out as a miaskitic system,
crystallizing titanite and ilmenite together with sodic-calcic amphibole and minor biotite. In interstitial
aggregates and pegmatitic patches representing trapped residual melts, the mafic and HFSE silicate
assemblages evolved through mildly agpaitic assemblages with sodic amphibole and pyroxene, astrophyllite
and aenigmatite, to highly agpaitic final assemblages with lorenzenite (Na2Ti2Si2O9). Throughout this
sequence, Zr remained contained in the NaFe2+0.5Zr0.5Si2O6 component in aegirine until hilairite
(Na3Zr(SiO3)3·3H2O) crystallized as the final magmatic mineral.
In contrast, in the green foyaite, eudialyte crystallized relatively early, together with aegirine and
aenigmatite. There are indications of high-Zr pyroxene cores, but zirconium content in aegirine dropped once
eudialyte started crystallizing. At a later stage, normandite (NaCa(Mn,Fe)(Ti,Zr)(Si2O7)2O2F2) crystallized.
There is no evidence that astrophyllite, lorenzenite or hilairite ever have formed in the samples investigated.
A chemographic analysis of melt-mineral equilibria suggests that the main driving force behind the
evolution towards more highly alkaline mineral assemblages in both rock types is indeed an increasing
degree of peralkalinity of the melt, probably driven by alkali feldspar fractionation (Elburg et al., 2017).
However, the contrast between the crystallization regimes of the white and green foyaite is controlled by
differences in water activity, the interstitial assemblages in the white foyaite having crystallized at higher
aH2O than the green. The trapped melts in the white foyaite evolved as closed systems until the end of
magmatic crystallization, whereas the green foyaite magma may have lost an aqueous fluid phase at an
earlier stage of evolution.
References:
Andersen, T., Erambert, M., Larsen, A.O., Selbekk, R. 2010. Petrology of nepheline syenite pegmatites in
the Oslo Rift, Norway: Zirconium silicate mineral assemblages as indicators of alkalinity and volatile
fugacity in mildly agpaitic magma. Journal of Petrology, 51, 2303-2325.
Elburg, M.A., Cawthorn, R.G., Andersen, T., 2017. Whole rock geochemistry of the Pilanesberg Complex:
Reflecting or controlling mineralogy? Abstracts, IMSG 2017
Retief, E.A., 1963. Petrological and mineralogical studies in the southern part of the Pilanesberg alkaline
complex, Transvaal, South Africa. Unpubl. D. Phil. thesis, Oxford Univ. 263 pp.
3 GRANULITE FACIES U, Th, REE OCCURRENCES IN THE NAMAQUALAND METAMORPHIC
COMPLEX: A REAPPRISAL
M.A.G. Andreoli1
1
School of Geoscience, University of the Witwatersrand, Johannesburg, South Africa;
[email protected]
Introduction. Over a period of more than 30 years the author was a geoscientist of the South African
Nuclear Energy Corporation and mapped, inspected and logged cores from numerous airborne/ground
radiometric anomalies within the Namaqualand Metamorphic Complex (NMC). Previously published results
of these studies are summarized as follows. Geology of U radiometric anomalies in the O’okiep Copper
district. In this area late stage, small intrusions, marginal to the batholitic ~1200 Ma U, Th-enriched
Concordia Granite, may constitute potential uranium resources (Robb, 1986; Andreoli et al., 2006, and ref.
therein). Additional radiometric anomalies are associated with members of the ~1030 ± 10 Ma charnockitic
Koperberg Suite that intruded under Lower-T granulite facies condition (~750-800 oC; Andreoli et al., 2006;
Meier et al., 2012, and ref. therein). The latter rocks include: a) Cu-barren but U, Th-enriched foliated
biotite diorites (B. Packham, pers. comm.); b) crosscutting dykes, veins of non-foliated, monazite-bearing
charnockite /pegmatite; c) Cu-mineralized, allanite, monazite, zircon-bearing norite; d) occasional bodies of
zircon-rich magnetite rock or nelsonite (Andreoli et al., 1994, 2006). Geology of Th, U radiometric
anomalies in the Vaalputs-Steenkampskraal area To the east of the town of Garies, a prominent,
irregularly shaped (~50 x 20 km) radiometric anomaly attracted past prospectors seeking economic uranium
and base metals deposits. At the local scale the anomaly was found to be rather discontinuous and related to
bodies of a) leucogranite; b) garnetiferous, ~1100-1066 Ma megacrystic, locally charnockitic orthogneiss; c)
pyroxene granulite (up to: U 50 ppm, Th 400 ppm; Nd 330 ppm); d) nelsonite in leucotonalite-anorthosite
dykes/plugs, and e) megacrystic, pegmatoidal charnockite veins (Andreoli et al., 2006). Prospecting near
Garies revealed bands of Th-REE rich nelsonite-leucotonalite; and near Kliprand intrusions of Th, Uenriched, locally monazite bearing nickeliferous gabbronorite-diorite (Maier et al., 2012). However, the
greatest mining potential rests with the dyke-like monazite-apatite orebody of the inactive Steenkampskraal
monazite mine, emplaced at ~1040 Ma and coeval (within error) to peak T conditions (~900oC; Kilian, 2011;
Maier et al., 2012; and ref. therein). In the mine and in satellite bodies the mineralization is often hosted by,
or in contact with a spectrum of coarse grained to megacrystic, often monazite-bearing intrusive rocks. The
latter include charnockite, alkali feldspar pegmatitic granite gneiss and other igneous rocks that are
plagioclase-dominated, with leuconorite, quartz diorite, enderbite, leucotonalite being most typical (Kilian,
2011; Andreoli et al., 1994; Reid et al., 2002). Geochemically, the ore is characterized by a negative Eu
anomaly and “bulk earth” Nd isotope ratios. Discussion. The body of data defines the NMC as an unusual
metallogenic province that evolved in 3 igneous pulses marked by U, Th enrichment and above average heat
production, at ~ 1200 Ma, ~1080 ± 20 Ma (~ Th/U = 0.5 - 23), and ~1040 Ma (~Th/ U = 10 - 100). Focusing
on the Steenkampskraal and related deposits, one model sources the monazite (+ Fe, Zr, S, Cu, Au) ore from
the partial melting of Th, REE-bearing metapelitic granulites at a lower crustal level (Kilian, 2011). The
second, preferred model considers the monazite-bearing charnockite veins/dykes and the related
monazite/nelsonite ores as highly fractioned end-members of mafic magmas extracted from metasomatized
subcontinental lithospheric mantle, to which the name Erlank Anomaly is applied (Andreoli et al., 1994,
2006; Maier et al., 2012). This model might link the Steenkampskraal-type and related Th, U-enriched mafic
Namaqua intrusions to the Caraiba, IOCG, and Kiruna-type deposits (Toll et al., 2016; Maier et al., 2012).
References. Andreoli, M.A.G. et al., 1994. Econ. Geology 89, 994-1016; Andreoli M.A.G. et al., 2006. J.
Pet. 47, 1095-1118; Kilian, J.G., 2011, B. Sc. Hon. Proj. Univ. Stellenbosch, 44 pp.; Maier, W. et al., 2012,
S. Afr. J. Geol. 115, 499-514; Reid, D. et al., 2002 Eur. J. Mineral. 14, 487-498; Robb, L., 1986, In:
Anhaeusser, C. (ed.) Mineral Deposits of Southern Africa II, Johannesburg, Geol. Soc. S. Afr., 1609-1627;
Troll et al., 2016, Abstr. Int. Geol. Congr. Cape Town https://www.researchgate.net/publication/308169128
4 CONSTRAINTS ON Cr-PGE MINERALISATION MODELS: GEOCHEMICAL AND
PETROLOGICAL STUDIES IN THE MIDDLE GROUP 1 AND 3 CHROMITITES,
WESTERN LIMB, BUSHVELD COMPLEX, SOUTH AFRICA.
Y. Arunachellan & S.A. Prevec
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected]
The preliminary results of a study on the sub-economic MG (Middle Group) layers within the CZ
(Critical Zone), contrasting the MG 1 (Lower CZ) and MG 3 (Upper CZ) chromitite layers of the
Rustenburg Layered Suite of the Bushveld Complex, South Africa are presented below.
The objectives of the study were: A) to determine if there was in-situ or proximal crystallisation of
the chromitite by evaluating mineral textures and compositions, B) to determine the characteristics of the
immediate HW (hanging-wall) and FW (footwall) to these chromitites, with insights into the relationship
that anorthositic zones may offer and C) to examine the PGE (platinum group element) profiles of the
chromitites in contrasting lithological settings. The sampled borehole is located in the Western Limb of
the Bushveld Complex the immediate HW, chromitite layers and FW were divided into sections (2.5
x 5 cm) along selected horizons for a microscale study.
The MG 3 chromitite layer is 101 cm thick with an immediate HW of 149 cm and a 150 cm FW. The
FW comprises of an anorthosite (110 cm) at the base followed by a leuconorite (10 cm) layer, a
chromitite stringer with undulating contacts (3 cm), an anorthosite (20 cm) then lastly to a 10 cm norite
which forms a gradational and straight contact to the MG 3 chromitite. The HW to the chromitite is a norite
(149 cm). The MG 1 chromitite is 61 cm thick with a 52 cm thick HW and FW. The FW has a
melanoritic (45 cm) layer followed by a 5 cm thick pyroxenite which then becomes a 3 cm leuconorite that
forms a gradational contact with the base of the MG 1 chromitite. The HW to the MG 1 chromitite is a 5
cm thick melanoritic layer with a gradational contact followed by a 3 cm thick pyroxenite, a 15 cm thick
melanonorite, a 20 cm gabbronorite and a melanorite of 10 cm forming the top of the sample range in the
HW. Both MG 3 and 1 silicates show disequilibrium textures between the pyroxenes and plagioclase, with
the formation of discontinuous olivine rims. These reaction rims are interpreted as products of magmatic
aqueous fluid reactions with minerals in a sub-solidus state. Deformation twinning of the plagioclase was
also noted in MG 1; this feature indicates either the transportation of these minerals or compaction by an
overlying crystal mush.
The compositions for the MG 3 package of plagioclase, pyroxene and chromite range from An67-78, En7186 and Cr# of 68-84 respectively. The MG 1 package of plagioclase, pyroxene and chromite compositions
are An64-91, En79-88 and Cr# of 70-80 respectively. The Cu/Pd ratio decreases from the base of the FW
as it approaches the base of the chromitite then remains depleted within the chromitite layer and finally
increases upwards in the HW. The trends are both significantly lower than MORB and are observed for
both the MG 3 and 1 package, there is no distinct trend in either the HW or FW adjacent to the chromitites.
The normalised PGE-Au shows relatively flat slopes in the HW and chromitites for both the packages but a
positive slope is observed in the FW showing PPGE enrichment relative to IPGE.
The preliminary geochemical and petrological data from the MG group study reveals that in-situ
fractional crystallisation seems unlikely as the sole mechanism for their formation in the CZ. The lack of a
stark difference in the PGE profiles along with contrast in the mineral chemistry and host lithologies for the
studied MG groups are likely indicators that a proximal crystallisation process and transportation
mechanism could form the basis for a genetic model.
5 ARCHAEAN ZIRCONS IN MIOCENE OCEANIC HOT-SPOT ROCKS ESTABLISH ANCIENT
CONTINENTAL CRUST BENEATH MAURITIUS
L.D. Ashwal1, M. Wiedenbeck1,2 & T.H. Torsvik1-4
1
School of Geosciences, Wits Univ., Private Bag 3, WITS 2050, South Africa; [email protected]
2
Deutsches GeoForchungsZentrum (GFZ), Telegrafenberg, D14473, Potsdam, Germany;
[email protected]
3
Center for Earth Evolution and Dynamics (CEED), University of Oslo, 0316 Oslo, Norway;
[email protected]
4
Geodynamics, Norges Geologiske Undersøklse (NGU), N-7491 Trondheim, Norway
A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian
Ocean island of Mauritius, on both the basis of inversion of gravity anomaly data (crustal thickness) and the
recovery of Proterozoic zircons (660-1971 Ma) from basaltic beach sands (Torsvik et al., Nature Geosci. 6,
227, 2013). We recovered 13 zircon grains from a trachyte associated with the Older Series basalts (9.0-4.7
Ma) of Mauritius, the second youngest member of a hot-spot track extending from the active plume site of
Réunion. Extreme care was taken to avoid contamination during sample processing. Ten of the 13 grains are
featureless, with no internal structures, and SIMS analyses (Cameca 1280-HR instrument) yield 49 spots
with Miocene U-Pb systematics and a mean age of 5.7 ± 0.2 Ma (1 sd), constraining the magmatic
crystallization age of the trachyte. Three grains with partially resorbed magmatic zoning, partial
metamictization and mineral inclusions (quartz, K-feldspar, monazite) show uniquely mid- to late-Archean
systematics: 20 spot analyses give concordant to near-concordant ages of 3030 ± 5 Ma to 2552 ± 11 Ma. This
suggests that during ascent, the trachytic magmas incorporated silicic continental crustal material that
preserves a record of several hundred m.y. of Archean evolution. This is consistent with Sr-Nd isotope
systematics of the Mauritian trachytes, which can be modelled as having been contaminated with 0.4-3.5%
of ancient granitoid crustal components. Our new age results, combined with the Proterozoic ages of zircons
recovered from Mauritian beach sands, are best correlated with continental crust of east-central Madagascar,
presently ~700 km west of Mauritius, where Archean gneisses and Neoproterozoic intrusive rocks are
juxtaposed such that a 2000 km2 area could correspond to a fragment of continent presently underlying
Mauritius. This, and other continental fragments formed during Gondwana break-up, may be scattered across
the western Indian Ocean. Some were later blanketed, and in the case of Mauritius, sampled, by plume
related volcanics.
Fig.1. Simplified geology of Madagascar and
India reconstructed to 90-85 Ma. Mauritius (M)
is reconstructed in a likely location near
Archaean-Neoproterozoic rocks in central-east
Madagascar just prior to break-up. The exact size
and geometries of Mauritius and other potential
Mauritian continental fragments (SM Saya de
Malha; N, Nazreth; CC, Cargados-Carajos Banks;
LAC, Laccadives; C, Chagos) are unknown. We
propose that Mauritia is dominantly underlain by
Archaean continental crust, and part of the ancient
nucleus of Madagascar and India (stippled black
line). AG, Analava gabbro (91.6 Ma); LR, Laxmi
Ridge; S, Seychelles; SM, St. Mary rhyolites (91.2
Ma). The black-white box shows a region of
Madagascar that could correspond to the Mauritius
zircons. Inset map shows simplified geology of
Mauritius, including trachyte plugs. Star symbol
marked MAU-8 is the sampling area for the
present study and black bars indicate locations of
zircons recovered from beach sand samples.
6 A DIFFERENCE IN 40Ar/39Ar AGE DATA BETWEEN THE MARGINAL ZONES OF THE
LIMPOPO COMPLEX
G.A. Belyanin1, T. Tsunogae2 & J.D. Kramers1
1
Department of Geology, University of Johannesburg, Johannesburg, South Africa; [email protected]
2
Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan
The Limpopo Complex (Belt) or Southern Africa is an internationally renowned Archean to Proterozoic
high-grade geological province [1]. Vast amount of geochronological data is now available for the
interpretation of its tectonic and metamorphic evolution. The Complex is divided into three major zones –
Northern, Central and Southern – and although they are structurally and petrologically related, the zones’
geochronological record is rather intricate [2]. The Northern Marginal Zone (NMZ) is largely located within
southern Zimbabwe and has not been a focus of major research over the last 15-20 years.
Our study reports new 40Ar/39Ar age results on amphiboles and micas separated from various rock varieties
over the large area in the NMZ and the adjacent southern part of the Zimbabwe craton. The majority of the
examined samples from all the three subzones of the NMZ (Triangle Shear Zone, Transition Zone and NMZ
s.s.) yielded ages in the range of ~1.94 to ~2.06 Ga, in full agreement with the previous studies [2], and no
reliable ages older than ~2.2-2.3 Ga were produced (Fig. 1a). Interestingly, several amphiboles from the
Buhwa Greenstone Belt (southern part of the Zimbabwe craton) yielded very convincing ages of ~2.0 Ga
(Fig. 1b). These results present a contrast to the argon-argon age data reported from the Southern Marginal
Zone [3], where both ~2.6-2.7 Ga and ~2.0 Ga records are prominent throughout the whole Zone.
[1] Kramers, J.D., McCourt, S. and Van Reenen, D.D. (2006) The Limpopo Belt. In M.R. Johnson, C.R. Anhaeusser,
R.J. Thomas, Eds., The geology of South Africa. Geological Society of South Africa. Johannesburg/Council for
Geoscience, Pretoria, 209-236. [2] Kramers, J. D. and Mouri, H. (2011) The geochronology of the Limpopo Complex: a
controversy solved. Geological Society of America, Memoir 207, 85-105. [3] Belyanin, G.A., Kramers, J.D, Vorster,
C., Knoper M.W. (2014) The timing of successive fluid events in the Southern Marginal Zone of the Limpopo
Complex, South Africa: Constraints from 40Ar–39Ar geochronology. Precambrian Research, 254, 169-193.
Figure 1. Argon-argon age spectra from amphiboles of the Northern Marginal Zone (a) and the Buhwa Greenstone Belt
(b).
7 THE PETROGENESIS OF THE TURFSPRUIT CYCLIC UNIT AND ITS HANGING WALL AND
FOOTWALL LITHOLOGIES AT TURFSPRUIT, NORTHERN LIMB, BUSHVELD COMPLEX,
SOUTH AFRICA
J.J. Beukes1, F. Roelofse1, C.D.K. Gauert2, D.F. Grobler3 & J.A.N Brits3
1
Department of Geology, University of the Free State, Bloemfontein, South Africa; [email protected]
2
Dezernat D23 – Angewandte Geologie und Georisiken, Landesamt für Geologie und Bergwesen SachsenAnhalt, Halle (Saale)
3
Ivanplats, 82 Maude Street, Sandton, Johannesburg, South Africa
The Platreef is defined by Kinnaird et al (2005) as “the lithologically variable unit, dominated by pyroxenite,
which is irregularly mineralized with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or
Archaean basement and the overlying Main Zone gabbronorites”. The principal host of Ni-Cu-PGE
mineralisation in the Platreef at Turfspruit located in the Southern sector of the Northern Limb is named the
Turfspruit Cyclic Unit (TCU) and is subdivided into a T1 (a non-mineralised, medium grained feldspathic
pyroxenite), T1m (mineralised pyroxenite at top of T1), T2U (mineralised pegmatoidal pyroxenite) and T2L
(mineralised pegmatoidal harzburgite and/or pegmatoidal olivine-pyroxenite) (Peters et al., 2014). The
proposed study investigates the petrology, whole-rock geochemistry and mineral chemistry of 4 boreholes
intersecting the TCU and its hanging wall and footwall lithologies in the ‘Flatreef’. The Flatreef is the flat to
gently dipping down-dip extension of the original Platreef discovery. The main aim of this study is to
determine and improve understanding of the processes involved in the TCU genesis. Lateral and vertical
variation of the orebody is determined by a) doing a comprehensive investigation of major and trace element
distribution of the dominant silicate minerals b) establishing the effect of country rock contamination and
assimilation on sulphide immiscibility and PGE scavenging as ore-forming processes on a microscale;
determination of mineral textures, chemical zonation and isotopy and c) to visualise the behaviour of
sulphide melt in the cumulus mush of the TCU in order to quantify the distribution of sulphides and possible
PGM in 3D by the use of X-ray Computed Tomography. Preliminary results indicate that the primary
minerals in the TCU has undergone varying stages of alteration. Intense serpentinisation occurred in the T2L
as olivine and pyroxene are poorly preserved. Where lithologies are not fresh, cumulus and intercumulus
plagioclase alter to sericite. Textural features such as exsolution lamellae of pyroxene, plagioclase inclusions
within orthopyroxene crystals, discontinuous rims of clinopyroxene and plagioclase zonation in the different
rock types indicate disequilibrium of the mineral phases. Well-rounded and corroded orthopyroxene crystals,
particularly in T1, suggest magmatic and chemical erosion of orthopyroxene crystals respectively. Sulphides
present consist dominantly of pyrrhotite, followed by pentlandite, chalcopyrite and to a lesser extent, pyrite.
These sulphides occur as disseminated, net-textured or as coarse blebs throughout the TCU. The sulphides
are mostly associated with secondary silicates. In the hanging wall and footwall the sulphides are relatively
less. The presence of talc, tremolite, chlorite, sericite and actinolite as alteration phases suggest that in
addition to crustal contamination large amounts of fluid alteration also played a key role in the petrogenesis
of the TCU.
References
Kinnaird, J. A., Hutchinson, D., Schurmann, L., Nex, P. A., & de Lange, R. (2005). Petrology and
mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa. Mineralium
Deposita, 40, 576-597.
Peters, B., Parker, H, Khul, T, Joughin, W, Lawson, M, De Swardt, G, Valenta, M (2014). Platreef 2014
Preliminary Economic Assessment (PEA), Limpopo Province, Republic of South Africa. Report prepared by
Orewin Pty Ltd for Ivanhoe Mines Ltd.
8 OLIVINE CRYSTALS FROM THE CENTRAL INTRUSION, ISLE OF RUM, SCOTLAND
J.E. Bourdeau1,2 & S.E. Zhang1,2 & A.D. Fowler2
1
Department of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;
[email protected] 2
Department of Earth Sciences, University of Ottawa, Canada.
Layered peridotites from the Central Intrusion feature cumulus olivine crystals that are encapsulated by large
oikocrysts of plagioclase and pyroxenes. Textural observations reveal that the olivine crystals display postcumulus reactions of a dissolution nature. By observing still-connected crystals and reconstructing larger
parental crystals, we notice that the reconstructed crystals are morphologically similar to spinifex olivine
crystals found in komatiites. The classic location of Pyke Hill, Canada, is chosen as a representative of the
spinifex olivine texture. Both olivine crystals from Pyke Hill and the Central Intrusion are physically and
chemically comparable. In addition, their host rocks share a similar geochemistry. Given these similarities,
we deduce that many aspects of the Central Intrusion were similar to that of komatiites. Hence, we propose
that phenocryst-free picritic melt periodically replenished Rum's sill-like magma chamber and a thermal
gradient, facilitated by a combination of a thin overburden, cold country rocks and/or circulating aqueous
fluids in the country rocks permitted the formation of spinifex-like crystals growing downward from the
roof. Subsequently, clusters of olivine crystals detached from the roof, aligned via hydrodynamic drag and
settled to form a near-horizontal foliation. Post-cumulus re-equilibration with infiltrating and percolating
intercumulus melts dissolved olivine crystals and fragmented the parental crystals into smaller entities.
9 USING DIHEDRAL ANGLES TO UNRAVEL THE FINAL STAGES OF UPPER ZONE
CRYSTALLISATION
Britt, T.L.1, Roberts, R.J.2 & Holness, M.B.3
1
University of Pretoria, Department of Geology; [email protected]
2
University of Pretoria, Department of Geology
3
University of Cambridge, Department of Earth Science
The final stages of crystallisation within layered intrusions tend to feature similar characteristics. The
mineralogy becomes increasingly more felsic with pyroxene and possibly olivine compositions decreasing in
Mg#, the increasing concentrations of more volatile and previously incompatible components and finally the
crystallisation of more evolved mineralogy namely apatite. These characteristics are brought about through
processes of fractional crystallisation and magma differentiation. Although the Upper Zone (UZ) of the
Bushveld Complex embodies exactly that, there are mineralogical and geochemical deviations that
complicate what should be the crystallisation of a single, felsic melt.
Early studies carried out by Wager and Brown (1968) suggested the subdivisions of the Upper Zone be based
on the appearance of new minerals along the Upper Zone stratigraphy. The first appearance of cumumlus
magnetite, olivine and finally apatite resulted in the Upper Zone being subdivided into UZa, UZb and UZc
respectively. Although these subdivisions are recognisable across all three limbs of the Bushveld Complex,
on a small scale, the appearance and sudden disappearance of a cumulus phase can take place over a scale of
a few centimetres. This feature is best represented by apatite.
The repetitive appearance and disappearance of apatite throughout the UZc implies that temperature-pressure
conditions varied episodically throughout the crystallisation of the Upper Zone. Due to the scale of these
variations, how these conditions were brought about requires a closer look into the thermodynamic
fluctuations as the magma was crystallising. The study of dihedral angles can help to determine the
thermodynamic evolution of the solid-fluid interface as the Upper Zone crystallised. With the use of this
data, dihedral angles could provide new insights into the finer scaled processes controlling the crystallisation
of the Upper Zone.
10 MORE THAN MEETS THE EYE: REMOTE SENSING OF THE KUNENE ANORTHOSITE
COMPLEX
Brower, A.M.1, Lehmann, J.2, Bybee, G.M.1, Corfu, F.3
1
School of Geosciences, Wits University, Private Bag 3 WITS 2050, South Africa;
[email protected] / [email protected]
2
Department of Geology, University of Johannesburg, Auckland Park, South Africa; [email protected]
3
Department of Geosciences and CEED, University of Oslo, Postbox 1047 Blindern, Oslo, Norway;
[email protected].
The Kunene Anorthosite Complex, located in south west Angola, is one of the largest Proterozoic anorthosite
intrusions on Earth, with an areal extent of at least 18 000 km2. Very little research has been conducted on
the Angolan portion of the complex and published maps lack detail and are often inconsistent. This study
makes use of interpretation of remote sensing datasets (Landsat 8 and SRTM – Shuttle Radar Topography
Mission) as well as U-Pb TIMS geochronology to analyse the magmatic architecture of the Kunene
Complex. In order to extract maximum compositional and structural data from this magmatic body, various
image processing techniques have been performed, including: false colour composites, a minimum noise
fraction, a principle component analysis and band rationing for the Landsat 8 data and, hill-shading and
automatic lineament extraction for the SRTM data. Using these techniques, in combination with ground
truthing, we have produced a new interpretative lithological map for the Kunene Complex and adjacent
country rocks. The results of Landsat Image processing enable identification of different spectral signals, and
allow us to differentiate the Complex from country rocks in addition to separating the Complex into different
compositional domains. From the SRTM imaging, lineament data were extracted and various structural
features can be seen throughout the complex. With a combination of ground truthing, these lineaments are
classified into either magmatic foliations, subsolidus planar structures and shear zones or fault structures.
This study reiterates the batholitic appearance of the Kunene Complex and identifies a possible framework
made up of 7-9 plutons. These plutons can be broadly classified both in terms of composition and age
groupings. The younger (~1380 Ma) more leucotroctolitic plutons occur to the north of a Red Granite belt
and the older (~1390 and ~1400 Ma) more leuconoritic plutons occur to the south of the belt. Our study
demonstrates the applicability of such types of remote sensing satellite mapping and image processing for
mapping studies in similar semi-arid environments.
11 EVIDENCE FOR MULTIPLE GRANITOID SHEET SOURCES IN HU SVERDRUPFJELLA,
ANTARCTICA
Burger, E.P. 1, Roberts, R.J.1, Grantham, G.H.2, Elburg, M.2, Ueckermann, H.2 and le Roux P3.
1
Department of Geology, University of Pretoria. Pretoria, South Africa. [email protected]
Department of Geology, University of Johannesburg, Johannesburg, South Africa.
3
Department of Geological Sciences, University of Cape Town, Cape Town, South Africa.
2
The HU Sverdrupfjella, in Western Dronning Maud Land, Antarctica is a mountain range between 72º S, 73º
S and 00º 35’ W, 01º 45’ E. HU Sverdrupfjella is composed predominantly of gneisses, but also includes
various intrusions. Among these intrusions are relatively thin granitoid sheets, these sheets are the subject of
this study.
During fieldwork 4 significant types of granitoid sheets were identified. These types were designated P0-P3
to reflect relative ages (using cross-cutting relationships), where P0 is the oldest.
The granitoids in this study were grouped into 4 phases and. The oldest P0 phase comprises sub-horizontal
folded sheets with axial planar foliations and was only observed at one locality.
P0’s are sub-horizontal folded granitic sheets with axial planar foliations and were only observed at the
Rootshorga nunatak. P1 granitoids are generally metaluminous pegmatites. The P1 phase is crosscut by the
P2 and P3 phases and has suffered some deformation, but displays no planar fabric, indicating that these
sheets are syn-tectonic. The P2’s are consistent with Dalmatian Granite, as defined by Grantham et al, 1991
[1], P2 granites are typically peraluminous. Field observations show that these granites intruded under brittle
conditions. The P3’s are cross-cut by P1’s and P2’s and typically pegmatitic. With the exception of grain size
P3’s are similar to P2’s and can safely be considered late and pegmatitic Dalmatian Granites.
Zircon U-Pb geochronology was done using LA MC-ICPMS at the University of Johannesburg.
Geochronology proved difficult due to poor suitability of zircons. However it can be demonstrated that P1’s
and P2’s are both of similar (Pan-African) age. Therefore granitoid sheets is HU Sverdrupfjella were
predominantly generated by the same event.
Using geochemistry; 3 distinct groups were identified bases on isotopic ratio’s and REE profiles; these
groups are interpreted to represent the melting of 3 distinct sources. These groups are designated S1-3. The
S1 group contains P0’s and some P1’s; the S2 group contains the bulk of the P2’s and S3 relates to P2’s.
The S1 group has a positive Eu anomaly and an inclined LREE gradient. S1 has a younger Sm-Nd model age
(`1000Ma) and higher εNd values (>-5). S2 has a fairly flat REE profile with a negative Eu anomaly and
lower εNd values than S1 (between -5 and -10).The S3 group has a steep LREE gradient and tends to have a
weak negative Eu anomaly. Has similar Sm-Nd model age to S2, but the lowest εNd values (<-10).
References:
[1] Grantham, G. H. et al. (1991) Antarctic Science. 3:197-204
12 THE ROLE OF SULFUR DURING PARTIAL MELTING OF ECLOGITE IN THE CRATONIC
MANTLE
S. Burness1, K.A. Smart1, G Stevens.2 & S. Tappe3
1
School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa;
[email protected]
2
Centre for Crustal Petrology, Department of Earth Sciences, Stellenbosch University, Private Bag X1,
Matieland 7602, South Africa
3
University of Johannesburg PO Box 524, Auckland Park 2006 Johannesburg, South Africa
The activity of volatile elements in the mantle (e.g. H, C) are known to have profound effects on the position
of the solidus [1] and on the composition and structure of melts. In particular, the presence of C can
drastically change melt character from silicate to carbonate [e.g. 2]. Metasomatism induced by fluids can
allow low-degree melting of the upper mantle at elevated pressures, producing economically-important
magmas such as kimberlites [3]. However, while the effects of carbon and water on melting in ultramafic and
mafic rocks at high-pressure are to some extent understood [e.g. 4], the effect of sulfur is relatively
unconstrained.
More recently, partial melting experiments in the presence of Sulfur (S) at upper mantle conditions suggest
that melting is likely a complex function of temperature, pressure and composition of both the sulfide [e.g. 5;
6] and the substrate (e.g. peridotite vs eclogite). Thus, in order to investigate the role of S as well as the
behaviour of highly siderophile and chalcophile elements during mantle petrogenetic processes, we are
conducting high temperature and pressure experiments on S- and C-bearing mafic bulk compositions at
upper mantle PT conditions. The focus of our study is to delineate the effect of S on the melting behaviour of
eclogite and the relationship between sulfide-melt formation relative to carbonate+silicate melt formation.
Our findings will add to the knowledge of volatile-bearing solidii in the upper mantle, the partitioning nature
of various major and trace elements between the silicate residuum, possible sulphide residuum, and possible
miscible or immiscible silicate-carbonate melts and sulphide melts. Comparison between the chemical
signatures of minerals produced in the experiments with those in natural samples is likely to shed light on the
processes of volatile-bearing metasomatism that may be linked to diamond formation.
[1] Green, D. H. (2015). Physics and Chemistry of Minerals, 42(2), 95-122.
[2] Wyllie, P.J. (1978). Geophysical Research Letters, 5(6), 440-442.
[3] Foley, S. F. (1990). Proceedings of the Indian Academy of Sciences-Earth and Planetary Sciences, 99(1),
57-80.
[4] Luth, R. W. (2003). Treatise on geochemistry, 2, 568.
[5] Bockrath, C., Ballhaus, C., & Holzheid, A. (2004). Science, 305(5692), 1951-1953.
[6] Li, Y., & Audétat, A. (2015). Geochimica et Cosmochimica Acta, 162, 25-45.
13 FLUID SOURCES IN THE TWANGIZA-NAMOYA GOLD BELT (SOUTH KIVU, DRC):
EVIDENCE FROM TOURMALINE COMPOSITIONS AND B- AND Rb-Sr ISOTOPE DATA
Steffen H. Büttner1, Johannes Glodny2, Thapelo Moloto1, Michael Wiedenbeck2, Gerald Chuwa3
1
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected]
2
Geoforschungszentrum Potsdam, Germany
3
Banro Corporation, Bukavu, DRC
The Twangiza-Namoya Gold Belt (TNGB) developed in a Proterozoic, variably composed meta-sedimentary
sequence that is associated with felsic and mafic igneous rocks. During polyphase tectono-metamorphic
episodes the basement was exposed to repeated periods of heat influx that mobilised hydrothermal fluids.
The most prominent thermal event is the emplacement of “G4” granites and pegmatites at the Meso- to
Neoproterozoic boundary, leading to the widely held assumption that Au mineralisation is granite-controlled.
However, the evolution of the TNGB hydrothermal/granitic system and its associated gold mineralisation are
poorly understood. Here we investigate the major elemental and the boron isotopic composition from
pegmatitic and hydrothermal tourmaline from the Kamituga and Lugushwa gold deposits as a proxy of
potential fluid provenance (Büttner et al., 2016).
Tourmalines from the hydrothermal veins are essentially dravitic with variable Ca contents. These
compositions are most readily affiliated with a variably Ca- and Al-rich pelitic source, such as the hosting
greenschist facies metasedimentary rock sequence. In contrast, the pegmatitic tourmaline is schorl-elbaite
dominated and largely Ca-free. Such tourmalines are characteristic for granitic environments.
The boron isotopic signature of hydrothermal tourmaline from the Kamituga deposit (δ11B: -18.4 to -15.3‰)
is distinctly lighter than that from Lugushwa (δ11B: -13.0 to -8.9‰). Pegmatitic tourmaline from Kamituga
shows intermediate δ11B values between -14.2 and -13.1 ‰ with an average of 13.9 ‰. The pegmatitic fluid
most likely had a δ11B composition in the range of -12.8 ‰. The isotopically significantly lighter boron in
tourmaline from hydrothermal veins from Kamituga cannot have formed by fractionation from this
pegmatitic source.
Using the Rb-Sr mineral isochron method we have dated the Kamituga pegmatite as 981±16 Ma, which
correlates in time with the regional late-Kibaran “G4” granites. Tourmaline-bearing veins from Lugushwa
are significantly younger and formed during the early stages of the Pan-African orogenic period (677±17
Ma; Rb-Sr mineral data).
The tourmaline major elemental compositions of hydrothermal tourmaline suggest a sedimentary source for
these veins rather than evolved granitic fluids. This is supported by boron isotope signatures that indicate that
boron in hydrothermal veins comes from an isotopically lighter source than the G4 granites. Furthermore, the
Pan-African age of one sample from Lugushwa indicates renewed hydrothermal activity post-dating the G4
event by ~300 m.y. Accordingly we suggest low-grade metamorphic fluid extraction from the sedimentary
host rock sequence as an important fluid source of the hydrothermal system in the TNGB. The gold bound to
the hydrothermal vein system therefore might also derive from this host rock, in which common black shales
may be a viable primary source. The Pan-African episode is a likely time at which this fluid and gold
extraction might have occurred. This however does not exclude granitic gold mineralisation that might have
taken place earlier, possibly related to the G4 granite emplacement.
References:
Büttner, SH, Reid, WK, Glodny, J, Wiedenbeck, M, Chuwa, G, Moloto, T, Gucsik, A (2016) Fluid sources
in the Twangiza-Namoya Gold Belt (Democratic Republic of Congo): Evidence from tourmaline and fluid
compositions, and from boron and Rb-Sr isotope systematics. Precambrian Research 280, 161-178.
14 A GLOBAL DATABASE OF DEEP CRUSTAL CUMULATES AND THEIR BEARING ON THE
EVOLUTION OF THE CONTINENTAL CRUST
Bybee, G.M.1, Chin, E.J.2 & Shimizu, K.3
1
School of Geosciences, Wits University, Johannesburg, South Africa; [email protected]
2
Scripps Institution of Oceanography, La Jolla, California, USA
3
Department of Terrestrial Magmatism, Carnegie Institution for Science, Washington DC, USA
Understanding how magmas evolve as they reach and transit the Moho – a complex zone of mass, heat and
chemical transfer between the crust and mantle – is crucial in models of crust formation and evolution.
Assessing processes operating in the crust-mantle transition zone using mid- to shallow-crustal intrusives or
volcanics can be difficult due to potential crustal assimilation. We have therefore assembled a global
database of over 600 published deep mafic and ultramafic cumulates across diverse tectonic settings
including mid-ocean ridges, ocean island hotspots, continental rifts, as well as island and continental arcs.
These rocks represent the fractionation products of magmas in these settings, and provide a new perspective
to investigate magmatic processes at the crust-mantle interface.
We find that mid-ocean ridge cumulates are Fe-poor (median FeOT = 6.28 wt%) and closely track MELTS
models of instantaneous cumulates formed from fractionation of a MORB-like melt at dry, low pressure
conditions. The Fe depletion in these cumulates is complementary to Fe-enriched MORBs that evolve along
the tholeiitic trend. In contrast, even the most primitive island and continental arc cumulates (Mg# >60) are
Fe- and Ti-rich (median FeOT = 9.20 and 10.90 wt%, respectively) and complementary to Fe-depleted arc
magmas evolving along the calc-alkaline trend. Our modeling and comparison with experimental data
indicate that fractionation of just 2 % magnetite can produce the observed calc-alkaline trend and Fe
depletion in the continental crust.
Cumulates formed beneath thicker crust (continental rifts, island and continental arcs) tend to track evolution
paths of accumulated cumulates (as opposed to instantaneous cumulates) and show more scatter. This may
be a natural demonstration of rheological strength changes predicted to occur with increasing thickness and
degree of differentiation in the crust (Mareschal & Jaupart, 2013, Tectonophysics 609, p. 524-534). We
propose that thicker and stronger crust, with a more pronounced rheological contrast, promotes prolonged
magmatism at the crust-mantle interface facilitating accumulation of cumulates as well as secondary
differentiation processes such as mixing between cumulates and through-going melts, homogenization and
crustal assimilation.
Although our results indicate that the thickness and strength of crust play some role in the evolution of
magmas at the Moho, given that both island and continental arc cumulates show enrichment in Fe and Ti,
crustal thickness and strength cannot be the only factors at play. Elevated water content and/or fO2 in arcs
probably also influences the extent of Fe-Ti-oxide fractionation. Thus, Fe-Ti oxide fractionation must exert a
primary control on the Fe-depletion and the calc-alkaline trend observed in arc environments. The
implications for crustal evolution are significant, as the upper continental crust displays Fe depletion and the
requisite, hidden, complementary reservoir may be represented by the Fe-rich cumulate xenoliths in our
study. Ultimately, through the crust-building processes of accretion and thickening, these Fe-rich cumulates
will transform into dense eclogitic phases and delaminate, producing observed andesitic bulk crustal
compositions.
15 ENIGMAS AND DEBATES ABOUT THE BUSHELD COMPLEX
R.G. Cawthorn
School of Geosciences, University of the Witwatersrand, PO Wits, 2050. [email protected]
I explore what we do not understand about the Bushveld Complex, highlighting debated issues, and
illustrating myopic interpretations that do not stand up to broader scrutiny. There are more questions than
answers.
1 Are (were) the east, west and north limbs connected? If so, from what level? What criteria define
connectivity?
2 When and why did the centripetal dip develop?
3 Where are the feeders?
4 Are models that assume an horizontal base valid? How dynamic was the sedimentary floor and how did it
affect the layering?
5 Did magma escape from the current limits of the Bushveld Complex? If so, to where?
6 What are the compositions of subsequent magmas? (There is considerable agreement on the first one.)
7 What are the dynamics of mixing?
8 How did such continuous layers form (in situ or settling)?
9 Does the formation of a monomineralic layer require the magma to be saturated only in that mineral?
10 Are significant proportions of grains introduced; or did the majority grow in the magma chamber?
10 Definitive layers are extremely uniform laterally, with one exception. (a) Why is the Pyroxenite Marker
discontinuous in the east and why do mineral compositions change laterally? What happens in the western
limb? (b) Is the troctolite layer in the northern limb an equivalent of the Pyroxenite Marker?
11 What do the “gaps” represent? Is the unconformity in the northern limb due to erosion?
12 Did layers accumulate in stratigraphic order, or did some intrude into the succession?
13 Did diffusion of elements and isotopes occur and what was the scale length?
14 Did the trapped liquid shift effect (TLSE) operate? To what extent can it be quantified?
15 How thick was the mush zone? How much compaction occurred?
16 (a) Did liquid immiscibility occur? If so, what rocks did it produce? (b) Are IRUPs immiscible liquids?
How did they intrude?
17 How long was the magmatic event in the main Bushveld area? What triggered it? What were the source
rocks? (What is the age of the amphibolite sills?)
18 How were the PGE concentrated?
19 What is the origin of the granitic rocks?
16 THE GIANT DONKERHUK BATHOLITH OF NAMIBIA: THE WORLD’S MOST
HETEROGENEOUS GRANITE?
J.D. Clemens, I.S. Buick & A.F.M. Kisters
Department of Earth Sciences, University of Stellenbosch, Matieland, South Africa; [email protected]
The syn- to late-tectonic, Cambrian (circa 520 Ma) Donkerhuk batholith intruded mainly pelitic schists of the
Southern-Zone accretionary prism of the Damara Belt. As a result of perhaps 20 Myr of crustal melting and
magma emplacement, the batholith is a very large, heterogeneous mass of mainly S-type granites assembled
from multiple, sheet-like magma batches. The highly heterogeneous source rocks are inferred to have been
arc metapelites, metapsammites and meta-andesites, with 87Sr/86Srt ranging from 0.7048 to 0.7206, and εNdt
from -13.6 to -1.8. Their Palaeoproterozoic Nd model ages fall between 1.75 and 2.35 Ga. Partial melting to
create the magmas of the batholith seems to have been largely localised in the metasedimentary part of the
source terrane, which was not the surrounding Kuiseb Formation. Both partial melting and magma
emplacement took place in the middle crust, with source rocks at depths ~ 22 km and emplacement at ~ 17
km. This small degree of magma ascent equates with a feeble degree of crustal differentiation, despite the
huge volume of magma transferred (perhaps 5000 km3). We have little evidence for magma evolution and
the variations in the rocks appear to be primary, source-related features.
The chemical and isotopic heterogeneity shown by this batholith seem to have no equal. We present the
following isotope correlation diagram as an illustration; the black dots represent the Donkerhuk batholith.
The fields for two other comparably large and relatively heterogeneous S-type batholiths are shown, for
comparison – the Leinster batholith, Ireland (Sweetman, 1987) and the Strathbogie batholith, southeastern
Australia (Clemens and Phillips, 2014). The Donkerhuk’s nearest rival for the title of most isotopically
heterogeneous granite is the Manaslu leucogranite in the Himalaya (Deniel et al., 1987), with greater Sr
isotope heterogeneity (87Sr/86Srt = 0.73789 to 0.76410) but far less Nd isotope variability (εNdt = -16.0 to
-13.0). However, the Donkerhuk has vastly greater major- and trace-element variability.
References
Clemens, J.D., Phillips, G.N., 2014. Inferring a deep crustal source terrane from a high-level granitic pluton:
the Strathbogie batholith, Australia. Contrib. Mineral. Petrol., 168, 1070.
Deniel, C., Vidal, P., Fernandez, A., Lefort, P., Peucat, J.-J., 1987. Isotopic study of the Manaslu granite
(Himalaya, Nepal) - inferences on the age and source of Himalayan leukogranites. Contrib. Mineral.
Petrol., 96, 78-92.
Sweetman, T.M., 1987. The Geochemistry of the Blackstairs Unit of the Leinster Granite, Ireland. J. Geol.
Soc. London., 144, 971-984.
17 THE CURIOUS CASE OF THE EXPLODING LAMPROPHYRE
P. Daya1, H.S.R. Hughes1,2, G.M. Bybee1, P. Horváth1, J.A. Kinnaird1 & T. Mbhele3
1
School of Geosciences, University of the Witwatersrand, Wits 2050, Johannesburg
Camborne School of Mines, University of Exeter, Cornwall Campus, Penryn, Cornwall, UK
3
School of Chemistry, University of the Witwatersrand, Wits 2050, Johannesburg
2
A suite of lamprophyric dykes cross-cut the Western Limb of the Bushveld Complex. These are reported as
being particularly hazardous to underground mining in some areas, either through falls of ground following
exposure via mining excavations, or by explosive gas outbursts associated with some dykes (Hughes et al.,
2016). One particular dyke outburst was associated with a very unusual lamprophyric composition. We
present the petrography and mineral chemistry of this dyke, alongside a novel experimental technique to
ascertain the gases produced by its degradation, with the aim of understanding the provenance of the gases
causing these lamprophyre-associated outbursts.
The dyke is composed of phlogopite and clinopyroxene set in a fine-grained groundmass of calcite, barite,
natrolite (a Na-zeolite) and halite, with accessory ilmenite and apatite. The phlogopite and clinopyroxene
contain complex compositional zonation from core to rim (e.g., for Ti and Mg) documenting magmatic
fractionation. Based on textural evidence, we find that the calcite, barite, zeolite and halite are secondary
minerals precipitated after replacement of the primary groundmass of the dyke.
We apply a method for the thermal decrepitation (under a nitrogen atmosphere to avoid combustion or
oxidation) with gas chromatography in order to thermally break-down the lamprophyre at a series of stepped
temperatures and to establish the emitted gas composition(s). We find that the dominant gases produced by
this lamprophyric lithology are carbon monoxide (CO), oxygen (O2) and water (H2O). The minor gas phases
produced by the lamprophyres are acetone (C3H6O), acetyl chloride (C2H3ClO), allyl acetate (C5H8O2),
carbon disulphide (CS2), methane (CH4), nickel tetracarbonyl (C4NiO4), and sulfur dioxide (SO2). Methane, a
common hazard in many South African mines, does not constitute a major gas emitted by the lamprophyric
dyke, contrary to assumptions made by the mine sites.
We discuss the possible processes governing the release of gases from the dykes. For example, by step
heating the lamprophyre sample we impose a phase change (akin to rapid depressurisation of the rock, as
would be caused by excavation underground) on the molecular species bearing volatile elements, and present
as fluid inclusions within the secondary minerals of the lamprophyric dyke (e.g., within halite, natrolite, etc).
In this way, the gas outbursts associated with the lamprophyres in the Western Bushveld may be analogous
to those experienced in salt mines (Molinda, 1988). Further, our step heating of the lamprophyric dyke at
higher temperatures (> 350⁰C) causes decrepitation of the constituent minerals themselves, and therefore
release of volatiles chemically bound into their crystal structure. Whilst it is unlikely that depressurisation of
the lamprophyres (via excavation) would cause this, it serves to highlight that the gases produced from these
higher temperature reactions are distinct from those of the lower temperature (< 350⁰C) analyses. We
suggest that the plethora of gases that can be produced from volatile-rich lithologies such as lamprophyres by
this experimental technique may aid in achieving an understanding of the underlying controls governing
these lamprophyric gas outbursts, and therefore be used in the prediction of gas-rich areas/dykes.
References
Hughes, H.S.R., Kinnaird, J.A., McDonald, I., Nex, P.A.M., Bybee, G.M. (2016). Lamprophyric dykes in the
Bushveld Complex: the lithospheric mantle and its metallogenic bearing on the Bushveld large igneous
province. Applied Earth Science (Trans. Inst. Min Metall. B), 125, 85-86.
Molinda, G.M. (1988). Investigation of Methane Occurrence and Outbursts in the Cote Blanche Domal Salt
Mine, Louisiana. US Department of the Interior, Bureau of Mines, 21pp.
18 IMPACTS, PSEUDOTACHYLITES, AND MELTING: THE ENIGMA OF THE DRURY
TOWNSHIP INTRUSION, CANADA
J.A. de Bruyn & S.A. Prevec
Department of Geology, Rhodes University, Grahamstown, P.O. Box 94, 6140, South Africa.
[email protected]
The Drury Township intrusion is believed to be one of a suite of ca. 2450 Ma Palaeoproterozoic
leucogabbroic intrusions associated with marginal rifting of the Superior Province craton. It is spatially
associated with a long-lived deep-crustal shear zone, and with an immediately proximal large impact melt
sheet. Existing U-Pb zircon geochronology (Prevec & Baadsgaard, 2005) indicates only impact-aged (ca.
1850 Ma) systematics, but the intrusion is cut by dykes thought to be members of the ca. 2220 Ma Nipissing
Diabase swarm, and is cut by extensive pseudotachylite broadly typical of those associated with the early
stages of emplacement of the impact melt sheet at 1850 Ma. The intrusion is transected by a post-impact
mylonitic zone. The zircon age is therefore believed not to represent the crystallisation age of the body,
although the zircons appear to be magmatic in origin.
The pseudotachylitic rocks are atypical of those associated with large impact craters (such as Sudbury and
Vredefort) inasmuch as they display a strong ductile fabric throughout, including along the contacts with
their leucogabbroic host rock. The pseudotachylites are also distinctive in that they host extensive in situ
melt veinlets, which appear to be pre-deformational, inasmuch as they cross-cut the deformational fabric, but
show evidence of strain. Melting is elsewhere present in the footwall breccias to the Sudbury impact melt
sheet, and is believed to be a product of footwall heating by the cooling melt sheet (e.g., Péntek et al., 2009).
The melt veinlets here are characterised, in part, by the presence of ribbons of quartz which display strongly
undulose (e.g., strained) extinction (Fig 1). The melt is otherwise fine-grained and shows no prominent
preferred orientation of crystals. Coarse-grains crystallising adjacent to the quartz ribbons display
characteristically euhedral development (Fig. 2), consistent with crystallisation in a melt.
1 cm
PT
Figure 1. Pink veinlet of melt (outlined) hosted by grey
pseudotachylite (PT) in coarse-grained deformed
metaleucogabbro.
Figure 2. Ribbon of recrystallized coarse-grained quartz
displaying undulose extinction and a euhedral amphibole
(photomicrograph in x-nicols).
References
Prevec, S.A. and Baadsgaard, H. (2005) Geochimica et Cosmochimica Acta 69, 3653 3669.
Péntek, A., Molnár, F., Watkinson, D.H., Jones, D.C. and Mogessie, A. (2009) International Geology
Review, 1–35.
19 GEOSPATIAL MAPPING OF SOUTH AFRICA’S LARGE IGNEOUS PROVINCE (LIP), SILL
AND DYKE RECORD
M.O. de Kock1, A.P. Gumsley2
1
Department of Geology, University of Johannesburg, Auckland Park, 2006, South Africa
2
Department of Geology, Lund University, Sölvegatan 12, Lund, 223 62, Sweden
LIPs, and their plumbing system of feeder sill provinces and dyke swarms provide temporal and spatial
constraints of the crustal units into which they intrude. The LIP, sill and dyke history of different cratons can
potentially be used to unravel the cratonic paleogeography during the assembly and breakup history
throughout the Supercontinent Cycle. LIPs have large spatial footprints that can continue across multiple
crustal fragments during continental break-up and dispersal. A now common first step towards the
reconstruction of former continents is the matching of short-lived LIPs using magmatic “barcodes”. Regional
dyke swarms and sill provinces can provide key information towards resolving exact paleocontinental
reconstructions, by providing geochronological, geometric and geochemical anchors and piercing points. The
paleomagnetic record of these magmatic events provide the only quantitative way to test paleocontinental
reconstructions, but the use of paleomagnetic poles is meaningless without evidence that the timing of
magnetic remanence acquisition in a rock equates to the actual rock age. U-Pb baddeleyite ages can now
routinely be determined from the analysis of just a few, or even single baddeleyite grain fractions. A large
quantity of new baddeleyite age constraints in recent years from major dyke swarms, sill provinces and LIPs
from across the Kaapvaal Craton has prompted this review. The review is complimented by an ArcGIS
database and map (Fig. 1). The purpose of the map is to highlight recent advances in the knowledge base,
and to identify key questions remaining to resolve the paleogeographic record of the Kaapvaal craton back in
time (particularly where the paleomagnetic record is concerned), and to reflect on our current understanding
of LIPs and there magmatic feeders.
Figure 1. Comparison of the dyke map of Uken and Watkeys (1997) with the South African LIP, sill and
dyke map (this study).
Uken R. and Watkeys, M.K. (1997) An interpretation of mafic dyke swarms and their relationship with major
mafic magmatic events of the Kaapvaal Craton and Limpopo Belt. S. Afr. J. Geol. 100(4), 341-348
20 DETAILS OF THE GABBRO–TO–ECLOGITE TRANSITION DETERMINED FROM
MICROTEXTURES AND CALCULATED CHEMICAL POTENTIAL RELATIONSHIPS
Johann F.A. Diener & Simon Schorn
Department of Geological Sciences, University of Cape Town, Cape Town, South Africa;
[email protected].
Permian-aged metagabbros from the eclogite type-locality in the eastern European Alps were partially to
completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed
along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro–to–eclogite
transition, allowing the details of the eclogitisation process to be investigated. Original, anorthite-rich
igneous plagioclase is pervasively replaced by fine-grained intergrowths of clinozoisite, kyanite and Na-rich
plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100–200 µm thick bimineralic
coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of
igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet.
Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases
towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become
progressively more jadeitic as eclogitisation proceeds. Given that the igneous plagioclase is pervasively
replaced by clinozoisite, kyanite and Na-rich plagioclase, whereas kyanite–diopside symplectites are
confined to narrow rim zones, we suggest that the development of these textures was controlled by the
(im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of
anhydrous plagioclase indicates that H2O diffusivity occurred on a mm-scale. By contrast, the size of the
anhydrous diopside–kyanite and diopside–garnet symplectites indicate that Fe–Mg–Ca–Na diffusivity was
limited to a 10s of µm scale. Chemical potential relations calculated in the idealised NCASH chemical
system show that the clinozoisite–kyanite–albite intergrowths formed due to an increase of µH2O to
plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid
conditions indicated by this texture span from virtually dry conditions (aH2O ~ 0.15) to H2O-saturation, and
therefore does not imply that the rocks were ever fluid-saturated. Calculations in the CMAS and NCFMAS
systems show that the gabbro–to–eclogite transition is characterised by the growth of garnet, diopsidic
clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a µCaO (+ Na2O)–µMgO (+
FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains.
The anhydrous nature of the textures indicate that the gabbro–to–eclogite transition is not driven by
hydration; however increased µH2O acts as a catalyst that increases diffusivity of all elements and rates of
dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that
crustal eclogite formation requires low H2O content, confirming that true eclogites are dry rocks.
21 MONAZITE U/Pb GEOCHRONOLOGY & GEOCHEMISTRY OF THE ORANGE RIVER
PEGMATITE BELT S.W. Doggart1, I. Buick1, D. Frei2, C. Lana3, P. Macey4, & C.W. Lambert4
1
Department of Earth Sciences, Stellenbosch University, Private Bag X1, 7602 Matieland, South Africa,
[email protected]
2
Department of Earth Science, University of the Western Cape, Private Bag X17, Bellville 7535, South
Africa; [email protected]
3
Department of Geology, Federal University of Ouro Preto, Minas Gerais, Brazil
4.
Council for Geoscience, 3 Oos str, Bellville, Western Cape, South Africa; [email protected]
Keywords: Namaqua, Pegmatites, monazite geochronology, isotope geochemistry
The pegmatites of the Orange River Pegmatite Belt (ORPB) form an extensive 16km wide, ca. 450km long
continuous W-E trending belt extending from Vioolsdrif to Kenhardt in South Africa, with the northern
extent reaching into the southern Karas region of Namibia. These pegmatites vary in composition and
internal structure, ranging from simple, homogenous bodies with quartz-feldspar-muscovite bearing
assemblages to complexly zoned, heterogeneous bodies that contain beryl, garnet, lepidolite,
columbitetantalite, together with U-bearing and REE—bearing mineral phases (zircon. monazite, xenotime,
apatite). The structural setting of the pegmatite belt is not yet well constrained regionally but has only been
locally described. Here, it was found that the pegmatites are concordant with the regional reworked D2 fabric
or in discordant D4 dilatational structural sites. Importantly, the ORPB has been linked to
tectonostratigraphic boundaries that subdivide the Namaqua sector of the Mesoproterozoic Namaqua-Natal
Province (NNP). The pegmatites of the ORPB are not linked with plutons at current exposure level.
This study attempts to explain the temporal and geochemical relationship of the pegmatites in the ORPB and
the lithologies of the Namqua sector of the NNP. Bulk rock geochemical analysis on pegmatites are largely
unfeasible due to their large grain size. However, REE-rich mineral phases (i.e. monazite, apatite) have been
proven to have an initial Sm/Nd isotopic composition equivalent to that of the bulk rock (Von Blankenburg,
1992; Tomascak et al., 1998; McFarlane & McCulloch, 2006). This study will use in-situ LA-MC-CP-MS
isotopic tracing of monazite grains to determine the Sm/Nd ratios and model ages for the pegmatites of the
ORPB. Geochronological data was obtained using in-situ high-spatial LA-Q-ICP-MS techniques to obtain
U/Pb age data for further constrain their emplacement ages.
The geochronological data obtained using the above methods yielded U/Pb dates ranging from ~960 Ma and
~1038 Ma concluding that the pegmatites of the ORPB were emplaced over a ~80 Ma period. Radiogenic
data obtained give ƐNd (1000) values of ~ -15 to ~ – 1, with the lowest values attributed to pegmatites within
the Richtersveld Magmatic Arc with ƐNd values increasing where pegmatites intrude into the more juvenile
Mesoproterozoic Kakamas Terrane.
Given their mineralogy (occurrence of beryl and exotic lithium phases such as spodumene or lepidolite) they
must represent some final stage fractionates of much larger felsic magma bodies. Additionally, the fact that
the ORPB pegmatites are observed in the field to intrude metasediments of the Namaqua-Natal Belt, might
be used to suggest that they were sourced via partial melting mechanism during late-stage D4 deformation of
these metasediments. This is hard to envisage owing to the fact that these granulites had undergone partial
melting and melt extraction at ~1.1 Ga. Hence were already refractory to further melting prior to the
emplacement age of the oldest (~1038 ma) of the ORPB pegmatites. The question is posed, how did these
late-stage, highly fractionated granitic rocks originate? What was their emplacement history?
22 WHOLE ROCK GEOCHEMISTRY OF THE PILANESBERG COMPLEX: REFLECTING OR
CONTROLLING MINERALOGY?
Marlina A. Elburg1, R. Grant Cawthorn2 & Tom Andersen1,3
1
Department of Geology, University of Johannesburg, South Africa; [email protected]
2
School of Geosciences, University of the Witwatersrand, South Africa
3
Department of Geosciences, Oslo University, Norway
The ca. 1.4 Ga Pilanesberg Complex consist of volcanic rocks that have been intruded by consanguinous
syenites and foyaites (nepheline syenite). The latter have been subdivided based on their texture and colour
by previous workers (e.g. Lurie, 1973), although exposure is poor and exact boundaries between the units are
difficult to define. Late-stage alteration has affected most of the rocks to a variable extent (e.g. Mitchell and
Liferovich, 2006).
Combining our new whole rock data with published analyses, we have divided the complex in different
geochemical groups, which roughly correlate with the previously defined units of the complex. In order to
minimise the influence of alteration, we have discarded analyses that have K2O/Na2O >0.8, as these have
suffered replacement of groundmass or nepheline by fine-grained muscovite. This excludes all analyses of
the Red Foyaite and tuffs. The rest of the data can be divided into a high (>50) and low (<50) Sr/Y group,
which also show little overlap in their Sr and Y concentrations. The low Sr/Y group contains all the samples
of Red Syenite, Ledig Foyaite and most of the White Foyaite; the high Sr/Y group contains the Green
Foyaite, Lujavrite and most of Tinguaites; dykes and lavas can belong to either group. Nearly all samples of
the high Sr/Y group are peralkaline, but only half of the low Sr/Y group, as the syenites are predominantly
metaluminous. Of the other trace elements only Ba and V (as well as the V/Ti ratio) show a clear distinction
between the two groups (higher in the high-Sr/Y group). Zr and Nb are well correlated, with the values only
slightly higher in the low- than in the high-Sr/Y group. Apart from the geochemical twins V and Ti being
decoupled from each other, Ga/Al is also quite variable, and correlates with peralkalinity.
When interpreting the whole rock geochemistry of intrusive rocks, the question whether the geochemistry
controls the mineralogy, or merely reflects it, is non-trivial. For instance, samples of Green Foyaite with high
Sr concentrations contain lamprophyllite (with >12% SrO and ca. 30% TiO2), which could also be a host for
V, so accumulation of this mineral could potentially explain the correlation between Ti, Sr and V in the highSr/Y group. However, we think that accumulative effects are less important than the inherent magma
chemistry, as the sparsely phyric extrusive rocks conform to the same trends. Moreover, accumulative effects
poorly explain why high-Sr samples would be Y-poor and vice versa.
In the high-Sr/Y group, an increase in peralkalinity is associated with an increase in Sr, Th, Pb, U, and
Na/K, coupled with a decrease in SiO2 from ca. 57 to 53 wt.%. This suggest that the increase in peralkalinity
is best explained as a result of fractionation of potassium feldspar, which is indeed one of the main
phenocrysts in the lavas. However, the fractionation of feldspar can only drive already peralkaline magmas
to higher peralkalinities, but not cause a magma to become peralkaline. The most likely way in which the
high-Sr/Y magmas reached peralkalinity is by amphibole-dominated fractionation from an alkali basalt ; for
the low-Sr/Y group, plagioclase may have played a role.
References:
Lurie, J., 1973. The Pilanesberg: geology, rare element geochemistry and economic potential. PhD Thesis,
Rhodes University, Grahamstown, 308 pp.
Mitchell, R., Liferovich, R., 2006. Subsolidus deuteric/hydrothermal alteration of eudialyte in lujavrite from
the Pilansberg alkaline complex, South Africa. Lithos, 91(1-4): 352-372.
23 A WHOLE ROCK AND Ta-Nb-Sn OXIDE GEOCHEMICAL STUDY OF THE CAPE CROSS – UIS
PEGMATITE BELT: ATTEMPING TO CONSTRAIN PETROGENEIC PROCESSES AND
MINERALISATION
1
Warrick C. Fuchsloch, 2Paul A.M. Nex, 3Judith A. Kinnaird
1, 2 and 3
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa.
[email protected]
2
[email protected]
3
[email protected]
1
The Pan African Damara Orogen, Namibia, is host to a variety of mineral deposits which include pegmatite
hosted lithium, tin, niobium-tantalum, uranium, and semi-precious stones (Miller, 1983[3]; Diehl, 1993[2];
Ashworth, 2013[1]). This paper is focussed on examining syn- and post-orogenic pegmatites ( ̴490 Ma;
mineralised and barren) of the Cape Cross–Uis pegmatite belt, to understand and give insight into
mineralisation controls and pegmatite evolution. The Cape Cross–Uis pegmatite belt is one of ten narrow
regional pegmatite belts, trending parallel to the orogen and lies within the Northern and South Kaoko Zones
of the tectonostratigraphic zones of the Damara Orogen. Within the belt, 3 major pegmatite swarms have
been identified (Diehl, 1993[2]); the Strathmore, Karlowa and Uis Swarms. These swarms include
cassiterite- and tantalite-bearing, unzoned, Uis-type tin pegmatites, and zoned lithium-beryllium pegmatites,
barren pegmatites and weakly zoned tourmaline-bearing, intragranitic pegmatites (groups 1, 2, 3 and 4
respectively)
Field mapping evidence shows that pegmatites intrude with a NNE trend, in line with the regional structural
fabric, along fold axes and bedding plane discontinuities. Pegmatites are variable in shape and size but are
generally elongate and stock-work like in form and range from 10 – 300 m in length. Thickness also varies
from about 1–70 m. Whole-rock major and trace element data have been acquired for pegmatites and granite
and metasediment host rocks. Granites show geochemical characteristics such as elevated TiO2, Ba, Y, Th,
U, and total REE values compared to negligible amounts (< 10 ppm) in pegmatite. Metasediment hosted
pegmatites show enrichment compared to granites in most other trace element values such as K2O, Ta, Sn,
Cs, Rb, Nb and peraluminousity. Geochemical evidence, such as fractionation trends, REE patterns and a
lack of pegmatite zonation surrounding granite bodies suggests that metasediment hosted pegmatites do no
share a parent daughter relationship with either Salem-type or Red/Grey granites. Metasediment host rock
interaction is evident within the pegmatite belt. Tourmalinised biotite schist contacts show appreciable
amounts of Ta, Sn and Nb. Whether or not this can be attributed to pegmatite fluids remobilising Ta, Nb and
Sn and concentrating them in greisenised areas (assimilation), is debatable. The reverse might be more
plausible, where late stage fractionated pegmatitic fluids rich in Ta, Nb, and Sn, reacted with host rocks.
Processes such as fractionation can be traced by whole-rock geochemistry as a definitive contributor to
element enrichment and a process of pegmatite evolution. Mineral chemistry data on Ta-Nb-Sn oxides where
acquired by microprobe. Data show that pegmatites reach a moderate fractionation level of the LCT beryltype pegmatites and that crystallisation occurred in a fluorine-poor environment. Ta/(Ta+Nb) values show a
vertical fractionation trend which is common of pegmatites with low F activity. It is likely that cocrystallising garnet and tourmaline control the Fe/(Fe+Mn) fractionation within the pegmatite. After the
crystallisation of tourmaline and garnet, late stage fluids become enriched in Ta and Mn and pegmatites
reach the highest fractionation trends observed. Studies of the zonation and mineral chemistry of coltan show
a complex crystallisation history and that supercritical fluids remobilised Ta and Nb in late stage greisens
[1] Ashworth, L. (2013). Mineralised pegmatites of the Damara Belt, Namibia. Fluid inclusion and
geochemical characteristics with implications of post tectonic mineralisation. PhD thesis (unpublished),
University of the Witwatersrand, Johannesburg.
[2] Diehl, B.T.M. (1993). Rare metal pegmatites of the Cape Cross – Uis pegmatite belt, Namibia. Geology,
mineralisation, rubidium strontium Characteristics and petrogenesis. Journal of African earth sciences, 17,
167–181.
24 A NEW ORIGIN FOR SUEVITIC BRECCIA DYKES IN PARAUTOCHTHONOUS IMPACT
CRATER FLOORS BY MECHANICAL MIXING OF PSEUDOTACHYLITE AND CATACLASITE
Roger L. Gibson, S’lindile S. Wela, Marco A.G. Andreoli
School of Geosciences, University of the Witwatersrand, PO WITS, Johannesburg 2050, South Africa
Suevite - a polymict impact breccia with particulate matrix containing lithic and mineral clasts in various
stages of shock metamorphism and cogenetic glassy or crystalline impact melt particles – is widely regarded
as one of the diagnostic lithologies produced by hypervelocity impacts. The 368-m-long M4 core located ~18
km NNW of the centre of the 145 ± 2 Ma Morokweng impact structure (South Africa) intersects highly
shocked and brecciated parautochthonous granitoid gneisses, metadolerite and dolerite containing an ~40 m
interval of melt-hosted breccia, as well as numerous cm- to m-wide dikes of suevite. Almost all breccias
show geochemical evidence consistent with derivation from the target rocks in their immediate vicinity.
Analysis of core samples and thin sections indicates that the suevite also occurs locally at the interface
between monomict lithic breccia and melt-matrix breccia in several composite dikes. This spatial
relationship and the similarities in composition and texture between the altered glass fragments in the suevite
and the altered glass matrix in the melt breccia, and between the cataclastic textures in the lithic breccia and
clasts in the melt-matrix breccia suggest that the glass fragments represent brecciated and plasticallydeformed, melt-matrix breccia. We propose that initial injection and variable quenching of thin, frictiongenerated, melt breccia dykes (pseudotachylite) into evolving fractures hosting lithic breccia was followed
by further movement along and across the hosting fractures that led to brecciation of the quenched portions
of the melt dikes and mechanical entrainment of melt fragments into the adjacent lithic breccia, producing a
hybrid, suevitic, breccia. Complex block movements during the modification stage of cratering leading to the
formation of the peak ring of the Morokweng crater provide both a generation and pumping mechanism for
active melt injection into the lithic breccia-hosting fractures and for ongoing post-quenching cataclastic flow
in the breccia matrix. In its essentially in situ derivation, this model differs from those proposed for crater fill
and fallout suevite and for dyke suevite in other craters that is ascribed to downward injection of crater fill
suevite into fractures in the crater floor, or brecciation and subsequent cataclasis of impact-melt dykes. 25 AR40/AR39 DATA FROM W. DRONNING MAUD LAND, ANTARCTICA: POSSIBLE
IMPLICATIONS FOR GONDWANA AMALGAMATION
Grantham G.H. 1, Kramers J. 1 & Burger E.P. 2
1
Department of Geology, University of Johannesburg, Auckland Park, Johannesburg 2006 South Africa;
[email protected]; [email protected]
2
Department of Geology, University of Pretoria, Pretoria, South Africa; [email protected]
Biotite and amphibole Ar40/Ar39 data on mineral separates from gneisses from the Sverdrupfjella and
Kirvanweggan areas of Maud Province of western Dronning Maud Land (DML), Antarctica are reported.
The samples were collected from “basement gneisses” the crystallisation ages of which are typically ~1000
and ~1200Ma old. The data provide insights into the post-orogenic evolution of western DML recognising
that the area was involved in the amalgamations of Rodinia and Gondwana at ~1000-1150 Ma and ~500600Ma respectively.
The data from the two areas show distinct differences. Data from six samples from Sverdrupfjella show that
the dates from Bt-Hbl are similar. The dates range between ~460Ma and ~550Ma except for one severely
disturbed Bt sample suggesting a date of ~326Ma. In four of the six samples the dates from Hbl are older
than those from Bt with differences between Hbl-Bt pairs of between ~20-40Ma whereas in a sixth sample
Bt yields a ~30Ma older date than Hbl.
In contrast, data from six samples from Kirwanveggan show greater variability. The dates from four of five
Bt samples range between ~498Ma-~520Ma with one sample yielding a date of ~868Ma. The dates from
seven Hbl samples range from ~480Ma to ~1260Ma. This range in age correlates very crudely spatially, with
the youngest dates being from northern Kirwanveggan, to the oldest from the most southerly basement
exposures in Kirwanveggan at Skappelnabben.
Immediately south of Skappelnabben, virtually undeformed quartz arenites and grits of the Urfjell Formation
with ages of ~530Ma (from whole-rock Rb-Sr data), ~566Ma (SHRIMP U/Pb maximum detrital zircon age)
and ~579Ma (Ar-Ar detrital muscovite age) are reported.
The data from Sverdrupfjella suggest a relatively short lived thermal pulse between ~460Ma and ~550Ma
with the narrow range between older Hbl and younger Bt pairs being consistent with rapid cooling. In
Sverdrupfjella and more broadly, DML, this age range is coincidant with widespread granitoid intrusion,
inferred to provide an advective heat source during this period. In contrast, in Kirwanveggan, where younger
granites are absent, the range in ages for Hbl are consistent with a crustal gradient between N. Kirwanveggan
(~5-6kb) and near surface at south central Kirwanveggan at Skappelnabben. The data are consistent with
deposition at surface of the Urfjell quartz arenites approximately ~550Ma ago.
The crustal gradient is consistent with post- orogenic erosional uplift of Sverdrupfjella and northern
Kirwanveggan with Sverdrupfjella having experienced thermal input from granitoid intrusions and is
consistent with the post -orogenic evolution of a mega-nappe collisional model for Gondwana amalgamation
proposed by Grantham et al. (2008) in which granitoid genesis is related to anatexis in the footwall of the
mega-nappe complex. The crustal gradient described above is consistant with thinning of the nappe complex
southwards and termination of the nappe north of the Urfjell area of Kirwanveggan.
References:
[1] Grantham, G. H. et al. (2008) Geological Society of London, Special Publications. 308: 91-119
26 EXTREME VARIATION IN QUARTZ PHENOCRYST δ 18O VALUE IN QUARTZ PORPHYRY
DYKES: IMPLICATIONS FOR PETROGENESIS AND INTRUSION DYNAMICS
C, Harris1 & K. Mulder1
1
Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
[email protected]
The 134 Ma Koegel Fontein igneous complex on the west coast of South Africa intruded during the initial
phase of break-up of Africa and South America. Quartz porphyry dykes that intruded gneisses that form the
roof of the main 25 km diameter Rietpoort granite, contain quartz phenocrysts that have extremely low δ18O
values (as low as -2.5 ‰). These values reflect those of the original magma, which were as low as -3.1‰,
some of the lowest magma δ18O values ever recorded. Samples were analysed from various points along two
NW-SE striking 10-15 m wide quartz-porphyry dykes that cross the roof zone of the granite (Fig. 1). Taken
together, the two dykes (A and B) traverse the entire roof pendant. These dykes are cut by the Rietpoort
granite at the NW end of A and the SW end of B and, therefore, predate the granite. Quartz phenocrysts in
the dykes have δ18O values that range from +1.1 to +4.6 ‰ (Dyke A), and -2.3 to +5.6 ‰ (Dyke B). There is
no systematic variation in δ18O value along the length of the dykes. Individual quartz phenocrysts from two
dyke samples have average δ18O value of -1.76 ± 0.51 ‰ (n=6), and +4.68 ± 0.62 ‰ (n=5), respectively.
Thus the variation in δ18O value within the quartz crystal population of individual dykes is small, and much
less than the lateral variation. Furthermore core and rim material from individual quartz phenocrysts in three
samples are identical within error; the phenocrysts are not zoned.
There is relatively little chemical variation in the dykes and such variations that exist do not correlate with
δ18O value. We suggest that the dykes are being fed by magma produced by partial melting of the same
mafic to intermediate source, which had been variably altered at high-temperature by 18O-depleted meteoric
water. Low- δ18O zones may have existed along veins or shear, but this has yet to be established. Magma so
formed stalled long enough in the crust to allow large crystals to form, yet did not homogenise. The δ18O
value of successive batches of partial melt would be expected to increase as melting progressed, because the
low-δ18O alteration zone(s) would have been progressively consumed. Variable quartz δ18O values can be
explained by vertical emplacement, at variable rates of ascent along the dyke. Thus the lateral variation in
quartz, and hence magma δ18O value at a particular location along the dyke would depends on the rate of
ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch. The Rietpoort
granite that post-dates the dykes varies little in δ18O value (average quartz = 8.2 ‰). The magmas evidently
became homogenised after the initial generation of low- δ18O magma, presumably after the low- δ18O zones
were consumed by the melt.
Figure 1. Quartz phenocryst δ18O values of samples taken along dykes A and B; the country rock is roof
pendant gneiss with the dykes being truncated at the NW and SE ends by the Rietpoort Granite.
27 MANTLE VISCOSITY AND THE SUPERCONTINENT CYCLE
C. Hatton1
1
Council for Geoscience, Silverton, South Africa; [email protected]
In the GyPSuM model (Simmons et al., 2010) mantle viscosity increases from 1020 Pa s near surface to 1023
Pa s at a depth of 1800 km. This increase is readily explained as the result of increasing viscosity with
increasing pressure. At depths greater than 2000 km mantle viscosity then decreases, to values below 1020 Pa
s at the core-mantle boundary. The low viscosity of the deep mantle is here related to the retention of
carbonate. Carbonate transforms from aragonite and post-aragonite structures to a tetracarbonate structure at
depths greater than 1770 km (pressures above 76 GPa; Pickard and Needs, 2015). In the aragonite and postaragonite structures carbon is in 3-fold co-ordination and carbonate cannot be retained in a silicate matrix,
where silicon is in 4-fold co-ordination. In the tetracarbonate structure carbon is in 4-fold co-ordination so is
retained in the silicate matrix. Carbonate in the bottom 1000 km of the mantle accounts for the reduced bulk
and shear acoustic wave velocities there (Marcondes et al., 2016). Carbonate allows plumes to rise from the
core-mantle boundary into the bottom 1000 km of the mantle. These low viscosity carbonated plumes reduce
the bulk viscosity of the deep mantle. The very high viscosity near 2000 km depth does not allow short term
ascent of plumes through this layer. Instead transfer is governed by the time scale of boundary layer
instability. Boundary layer instability can be quantitatively related to viscosity (Turcotte and Schubert,
2002). For a viscosity of 1023 Pa s the time scale of boundary layer instability is 1.02 billion years. Because
this lies in the range of the Sutton cycle of repeated formation and breakup of supercontinents (Hoffman,
1999), the supercontinent cycle is here related to the episodic disruption and ascent of a thermal boundary
layer from a depth of 2000 km. These episodes result in the reorganisation of the lower mantle convection
system, switching it through 90° (Pavoni, 1985), as in the orthoversion model of Mitchell et al. (2014).
Currently the centres of lower mantle upwelling are situated below the African and Pacific plates, driving the
continents of the disrupted Pangean supercontinent into a girdle encompassing the American and Asian
continents. Five supercontinent cycles have been completed. The first cycle began in the Hadean at 4467 Ma
and ended with the assembly of an Africa- centred supercontinent at 3850 Ma, the Hadean-Archean
boundary of Bleeker (2004). This supercontinent was destroyed by mixing into the mantle to provide the
sources for the EM I and EM II components of Zindler and Hart (1986). The second cycle began in the
Paleoarchean and ended with the assembly of an America-centred continent, Kenorland at the end of the
Mesoarchean. The onset of Neoarchean Ventersdorp-related magmatism at 2785 Ma defines the beginning of
the third cycle, which ended with the assembly of the African-centred supercontinent, Columbia, defining the
end of the Eoproterozoic. Bushveld magmatism at 2055 Ma marks the beginning of the Paleoproterozoic and
the breakup of Columbia at the start of the fourth supercontinent cycle, which ends in assembly of Rodinia
by 1000 Ma. The fifth supercontinent cycle ended with the assembly of Pangea at 252 Ma.
Bleeker, W. (2004). Towards a ‘natural’time scale for the Precambrian–A proposal. Lethaia, 37(2), 219-222.
Hoffman, P. F. (1989). Speculations on Laurentia's first gigayear (2.0 to 1.0 Ga). Geology, 17(2), 135-138.
Mitchell, R. N., Kilian, T. M., & Evans, D. A. (2012). Supercontinent cycles and the calculation of absolute
palaeolongitude in deep time. Nature, 482(7384), 208-211.
Marcondes, M. L., Justo, J. F., & Assali, L. V. C. (2016). Carbonates at high pressures: Possible carriers for
deep carbon reservoirs in the Earth's lower mantle. Physical Review B, 94(10), 104112.
Pavoni, Nazario (1985). Die pazifisch-antipazifische Bipolarität im Strukturbild der Erde und ihre
geodynamische Deutung. Geologische Rundschau. 74 (2): 251–266.
Pickard, C.J. & Needs, R.J. (2015) Structures and stability of calcium and magnesium carbonates at mantle
pressures, Physical Review B, 91, 104101. Simmons, N.A., Forte, A., Boschi, L., Grand, S., 2010. GyPSuM:
a joint tomographic model of mantle density and seismic wave speeds. J. Geophys. Res. 115 (B12310)
Turcotte, D. L. & Schubert, G. (2002). Geodynamics, 2nd ed. 456 pp. Cambridge
Zindler, A., & Hart, S. (1986). Chemical geodynamics. Annual Review of Earth and Planetary Sciences, 14,
493-571.
28 SYN-BUSHVELD DIORITE SHEET INTRUSIONS IN THE MAIN ZONE: A RECORD OF
CRUSTAL MELT TRANSFER?
B. Hayes1, G.M. Bybee1 & P.A.M. Nex1
1
School of Geosciences, Wits, Johannesburg, South Africa; [email protected]
Bushveld-aged diorites have been documented in localities peripheral to the Bushveld Complex [1] and
olivine-bearing diorites are present in the Upper Zone of the Complex [2]. We document pegmatitic diorite
sheets that are intruded into Main Zone cumulates in the western Bushveld. Field relationships, including a
lack of chilled margins as well as diffuse to sharp contacts with the country rock, indicate that the diorite
sheets were coeval with Bushveld magmatism. U-Pb zircon geochronology reveals a Bushveld-age (2054
±11 Ma), corroborating the field evidence. The diorite sheets are zoned, with mesocratic and leucocratic
regions. Mesocratic regions contain plagioclase and hornblende. Accessory phases include biotite, pyroxene,
chlorite, actinolite, magnetite, apatite, zircon and galena. Occurrences of fine-grained pyroxene included in
hornblende and biotite cores suggest partial ingestion of gabbroic cumulates or peritectic reaction (e.g.
pyroxene + melt = hornblende). Leucocratic regions are dominated by quartz and K-feldspar, which develop
graphic textures. Mineral chemical data reveals that plagioclase cores are dominantly of andesine
composition (av An50). Locally, plagioclase cores are more primitive (~An70) and are in equilibrium with
cumulus plagioclase in the host Main Zone gabbronorites. Plagioclase margins in the diorite sheet are
strongly normally zoned (~An25), indicating an evolution to more evolved melt compositions. Limited
pyroxene data from inclusions in hornblende show that pyroxene is in near equilibrium with host Main Zone
pyroxene (~Mg# 60). Equilibrium between plagioclase cores and rare pyroxene grains in the diorite sheet
with the surrounding Main Zone suggests they may be partially digested relicts of Main Zone cumulates.
Invading diorite melt would have infiltrated incompletely solidified Main Zone gabbronorites as sheet
intrusions, interacting with local cumulates by reactive porous flow. Si-rich melts reacted with pre-existing
pyroxenes to produce peritectic hornblende. Extensive hornblende crystallization led to further Sienrichment of the residual melt (as well as K-enrichment) to promote quartz, K-feldspar and biotite
crystallization. Biotite tends to form overgrowths on hornblende suggesting a peritectic reaction. Quartz and
K-feldspar form local concentrations resulting from pooling of Si- and K-rich melts. Minor sulfides suggest
that the late evolved melt reached sulfur saturation, possibly driven by fractional crystallization.
Thermometric calculations using plagioclase and hornblende equilibrium pairs indicate crystallization
temperatures in the range 870-720ºC [3]. A key question is the origin of the diorite melts. These melts may be
accumulations of residual liquid after extensive crystallization of the Main Zone, indicating the incomplete
extraction of melt from the cumulate pile. Diorite melts may have injected downwards from the Upper Zone
where diorites are known to occur in the layered sequence. However, models using the MELTS algorithm for
putative Bushveld liquid compositions fail to reproduce the observed dioritic phase assemblage.
Alternatively, diorite melts may be derived from crustal melting associated with the wider Bushveld
magmatic event. A crustal origin is supported by low Th/U in zircon. Crustal melts may have been
transferred through a network of sheet intrusions throughout the Bushveld Complex, eventually feeding the
overlying granites.
[1] de Waal, S.A. & Armstrong, R.A. (2000), The age of the Marble Hall diorite, its relationship to the
Uitkomst Complex, and evidence for a new magma type associated with the Bushveld igneous event. South
African Journal of Geology, 103, (2), 128-140. [2] von Gruenewaldt, G. (1971), A petrographical and
mineralogical investigation of the rocks of the Bushveld igneous complex in the Tauteshoogte-Roossenekal
area of the eastern Transvaal. PhD Thesis. [3] Holland, T.J.B. & Blundy, J.D. (1994), Non-ideal interactions
in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy
& Petrology, 116, 4, 433-447.
29 EXPERIMENTAL CONSTRAINTS ON CRUSTAL CONTAMINATION IN PROTEROZOIC ANORTHOSITE PETROGENESIS C.M. Hill1, G.M. Bybee1, L.D. Ashwal1 & S.W. Parman2
1
School of Geoscience, University of the Witwatersrand, Johannesburg, South Africa;
[email protected]
2
Department of Earth, Environmental and Planetary Science, Brown University, Providence, USA.
Massif-type anorthosites are characterized by their large volume, plagioclase-dominated mineralogy
(up to 90% An40-60) and restriction to the Proterozoic Eon (0.6 to 2.7 Ga). The composition of the parental
magma is believed to be high-Al basalt, but whether this magma originated from partial melting of the
mantle or the crust is debatable. In many massifs, including the ~18,000 km2 Kunene Anorthosite Complex
(KAC) in Angola, distinct plutons of orthopyroxene-bearing and olivine-bearing anorthosites can be
distinguished. This mineral dichotomy has been attributed to variable degrees of crustal contamination of a
mantle-derived magma. However, previous experiments and modeling have shown a thermal divide on the
plagioclase-pyroxene liquidus surface, that causes olivine-normative liquids to evolve to lower Si
compositions. The thermal divide implies that partial melting of a mantle source cannot produce the parental
magmas of anorthosites, and that the observed range of mineral assemblages in the rocks cannot be explained
by crustal contamination of mantle-derived magma at 10-13 kbar (Longhi et al., J. Petrol. 40, 1999; Longhi,
Lithos, 83, 2005). These models propose that lower crustal anatexis and subsequent fractionation would
produce a closer match to the observed mineral compositions.
We have conducted piston cylinder experiments at 10 kbar using a high-alumina basaltic starting
composition, as well as a mixture of the composition with a coeval granitoid from the KAC, to model the
effects of contamination on phase equilibria relevant to anorthosites. The experiments show a shift from
olivine to orthopyroxene in the crystallizing assemblage following contamination, and also do not show a
thermal divide. The results indicate that the parental magmas of anorthosites could be derived from melting
of the mantle, and that the olivine-orthopyroxene dichotomy can be produced by crustal contamination of
mantle-derived, high-alumina basaltic magma at depth.
Fractionation and contamination modeling using MELTS (Smith and Asimow, Geochem, Geophys.
Geosyst. 6, 2005; Ghiorso et al. Geochem, Geophys. Geosyst. 3, 2002) reproduces the liquid evolution trends
of the experiments well and so we use it to model the effects of variable petrogenetic conditions. The
modeling and experimental results are compared with new petrologic and geochemical data from the KAC to
show that crustal contamination of mantle-derived melts is a crucial process in Proterozoic anorthosite
petrogenesis.
30 THE GEOCHEMISTRY OF THE LAYERED MOLOPO FARMS COMPLEX, SOUTH AFRICA:
PRELIMINARY RESULTS.
R. Peace Hlungwani1 & M.A. Elburg1
1
Department of Geology, University of Johannesburg, Auckland Park, South Africa: [email protected]
The layered Molopo Farms Complex (MFC) is located on the border of Botswana and South Africa, 200 km
west of the Bushveld Layered Complex (BC). It is proposed that the MFC is similar to the BC with regards
to rock types, age, geological setting and possible mineralization (Prendergast, 2012). However, the
geochemistry of the MFC is poorly known. The aim of this project is to characterise the MFC in terms of its
whole rock chemistry, mineral chemistry and plagioclase Sr-isotope composition, and compare it to the BC.
Samples were collected from boreholes from the South African part of the MFC. The rocks of the MFC are
layered and have a cumulate texture similar to those of the BC. Samples from borehole MOM 2
(gabbronorites, gabbro and serpentinised pyroxenite) are altered, as evidenced by the presence of amphibole
and chlorite, and sericite after plagioclase. The MOM2 samples have Mg# values ranging from 28 to 62,
which are low compared to gabbronorites of the BC Main Zone (Roelofse and Ashwal, 2012), and more
similar to those of the Upper Zone Subzone C (Tegner et al., 2006; Cawthorn, 2013). The An values vary
widely between An55 and An83. Samples from MOM 3 (gabbronorites, gabbro, serpentinite and pyroxenite)
contain orthopyroxene with exsolution lamellae and fresh plagioclase. The samples have Mg# values are
ranging from 53 to 70, which are similar to those of the Upper Zone (Tegner et al., 2006). The An values are
An44-An88 and orthopyroxene is En49-En70. Major element compositions of MOM 2 and MOM 3 overlap
with data from the Upper Zone (Cawthorn, 2013) with the exception of MgO and P2O5. Most samples from
MOM 7 are completely altered to serpentinite, but pyroxenite layers are partially altered. Cumulate textures
can still be observed where olivine, now serpentine, is the cumulus mineral, suggesting that the serpentinite
protolith was probably harzburgite, this is based on the Mg# values of 83-87 which are similar to those from
Wilson (2015). MOM8, which contains cyclically layered pyroxenite and harzburgite, and can be correlated
to the lower zone, based on similar layering and rock types. The petrography shows that the pyroxenes
(>90%) (both clinopyroxene and orthopyroxene) are cumulus and plagioclase (<10%) is interstitial. The
major element whole rock chemistry of the orthopyroxenites is similar to those of the BC (Wilson, 2015).
The harzburgites of the MFC have a lower silica content than those of the BC. This may reflect a higher
olivine content, or alteration. The Mg# values (79-88) of whole rock are similar to those of those BC. The
data collected thus far is in agreement with the idea that the Molopo Farms Complex is similar to the
Bushveld Complex, however, further studies such as Sr-isotope on plagioclase minerals will be conducted in
order to further confirm the idea of the MFC being one of the BC limbs.
References
Cawthorn R.G. (2013). The residual or Roof Zone of the Bushveld Complex, South Africa. J. Petrol., 54,
1875-1900.
Prendergast, M.D. (2012). The Molopo Farms Complex, southern Botswana– a reconsideration of structure,
evolution, and the Bushveld connection. South Africa J. Geol. 115, 77-89.
Roelofse, F. & Ashwal, L.D. (2012). The Lower Main Zone in the Northern limb of the Bushveld Complexa >1.3 km thick sequence of intruded and variably contaminated crystal mushes. J. Petrol., 53,14491476.
Tegner, C., Cawthorn, R.G. & Kruger F.J. (2006). Cyclicity in the Main and Upper Zones of the Bushveld
Complex, South Africa: Crystallization from a Zoned Magma sheet. J. Petrol. 47, 2257-2279
Wilson, A.H. (2015). The earliest stage of emplacement of the eastern Bushveld Complex: Development of
the Lower Zone, Marginal Zone and Basal ultramafic sequence. J. Petrol. 56, 347-388.
31 CALCULATING P-T CONDITIONS FROM GARNET MICA SCHISTS, TUGELA TERRANE,
SOUTH AFRICA
P. Horváth1 & J. Reinhardt2
1
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa,
[email protected]
2
Department of Geology, University of the Western Cape, Bellville, South Africa, [email protected]
The Tugela terrane forms the northernmost section of the Mesoproterozoic Natal Metamorphic Complex,
exposing medium- to high-grade metamorphic rocks for some 300 km in a N-S direction, close to the eastern
coast of South Africa. The Tugela terrane has been thrust onto the southern margin of the Archaean
Kaapvaal Craton and is separated from this craton by a narrow zone of low-grade rocks. Field evidence from
the Tugela terrane points to dominantly amphibolite-facies conditions in all four of the thrust sheets
comprising the bulk of this terrane. Three of the four sub-units as defined by Matthews & Charlesworth [1]
consist essentially of felsic and mafic metamorphic rocks of igneous origin. The fourth, so-called Madidima
sheet shows a larger compositional range that includes felsic gneisses, amphibolites and a variety of
supracrustal rocks. Except for an early descriptive paper by Cain [2] and a more recent study by Bisnath et
al. [3] on amphibolites specifically, there are virtually no metamorphic-petrological data available for this
area. Thus our study on the staurolite- and sillimanite- garnet mica schists within Madidima sheet is the first
to provide P-T data based on metapelites for the Tugela terrane.
The analysed staurolite-garnet mica schists show compositional layering on a hand-specimen scale, with
alternating biotite-plagioclase-richer and garnet-staurolite-richer layers. The former layers have higher MgO
and Na2O contents, but lower Al2O3, FeO and K2O contents compared to the latter ones. According to
petrographic observations, chlorite is not a retrograde phase in the mica schists, but rather forms part of the
peak metamorphic assemblage with garnet, staurolite, biotite, muscovite, plagioclase and quartz. Biotite-free
mica schists from the same exposure have higher SiO2, lower MgO, FeO and CaO contents compared to
biotite-bearing ones. Quantitative phase diagrams were constructed in the MnNCKFMASH and
MnNCKFMASHTO systems using THERMOCALC. The results show similar P-T conditions for the biotitebearing and biotite-free mica schists: 530-580 ◦C and 3.2-5.7 kbar for MnNCKFMASH, and 520-550 ◦C and
3.1-5.3 kbar for MnNCKFMASHTO, respectively. Calculated and measured mineral compositions match
fairly well for garnet, plagioclase and staurolite in both systems, but discrepancies for biotite and chlorite
compositions have been noted. Sillimanite-garnet mica schist samples from a separate outcrop, just over 3
km from the staurolite-chlorite-bearing schist locality appear to indicate a slightly higher grade of
metamorphism, possibly pointing to tectonic discontinuities within the Madidima thrust sheet. Sillimanite
appears only in the foliated matrix together with biotite, muscovite, quartz and rare plagioclase, but never as
an inclusion phase in garnet. Inclusion-rich cores and inclusion-poor rims in garnet indicate multiple garnet
growth events, a Grs “spike” is present in the garnet zoning profiles at the boundary of the two areas, which
is accompanied by a decrease in Alm and XFe [Fe/(Fe+Mg)] content without significant changes in Prp and
Sps components. The cores have fairly flat zoning profiles, whereas the rims show increasing Alm and XFe
with decreasing Prp compositions. Staurolite is present in the inclusion-rich zones (and rarely in the
inclusion-poor rims), but never in the matrix, indicating that garnet growth started in the presence of
staurolite, but ended outside its stability field, in the sillimanite field. These observations are supported by
quantitative phase diagram calculations in the MnNCKFMASH system. We calculated 600-640 ◦C and 5.76.3 kbar for the formation of the garnet inclusion-rich core. Continuous growth of garnet (marked by the
inclusion-poor rims) occurred during near isothermal decompression in the garnet-staurolite field, while the
garnet edges and matrix assemblage yielded 600-630 ◦C and 4-4.5 kbar in the garnet-sillimanite field.
References:
[1] Matthews PE & Charlesworth EG (1981) Northern Margin of the Namaqua-Natal Mobile Belt in Natal,
National Geodynamics Project. (1:140 000 geological map). [2] Cain AC (1975) Geol Rundschau 64: 192216. [3] Bisnath A et al. (2008) South African J Geol 111:369-386.
32 ORIGIN OF MANTLE-DERIVED CARBONATE NODULES FROM THE BULTFONTEIN
KIMBERLITE
G.H. Howarth1, A.E. Moore2, & C. Harris1
1
Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
2
Geology Department, Rhodes University, Artillery Road, Grahamstown, South Africa
In order to account for CO2-rich, Si-undersaturated magmas such as carbonatites and kimberlites, it has long
been suggested that a carbonate-bearing peridotite mantle source must be present in the upper mantle.
Experimental evidence suggests that if carbonate was present within kimberlite-derived mantle xenoliths, it
would dissociate and degass and should not survive transport from the sub-continental lithospheric mantle
(SCLM) to the surface. However, several studies have described the presence of fine-grained (10-20 µm)
mantle-derived carbonate. Carbonate also occurs as inclusions in ilmenite in a number of different
kimberlites. This suggests that in some cases carbonate may indeed survive rapid transport to the surface
during kimberlite ascent. Here, we present major and trace element data along with stable isotope data for
coarse-grained (>1 cm) carbonate nodules from the Bultfontein kimberlite, South Africa. The chemistry of
the carbonate indicates a mantle origin: high-Sr and -Ba concentrations and mantle-like δ13C (~6 ‰). The
exception being δ18O, which has been affected by late-stage deuteric alteration resulting in elevated values
(~16 ‰); a common feature of kimberlitic carbonates. Carbonate stable within the mantle has been shown
experimentally to be dominantly dolomite and magnesite; however, the carbonate nodules here are calcite.
Textural evidence indicates that these calcite nodules are not in equilibrium with the surrounding kimberlite,
a feature observed for other mantle xenoliths/xenocrysts; e.g., garnet. The margins of the nodules are
characterised by the presence of quench-related textures in the form of radiating clusters of microlites, a
glass phase, and spherulitic structures. The textures indicate quenching of a liquid at the interface between
the nodules and the kimberlite melt. Olivine within the sample is generally very fresh relative to typical
kimberlites; however, where olivine is observed nearby the nodules they are partially altered. This indicates
the loss of a fluid phase from the nodule and resultant alteration of the surrounding olivine. Two possible
interpretations can be suggested for the origin of the carbonate nodules: 1) Late-stage carbonate segregations
as part of the crystallising kimberlite melt, a common feature in most kimberlites, or 2) mantle-derived
carbonate transported to the surface by the kimberlite magma, much like peridotite and eclogite mantle
suites. The large size (>10 mm) of some of the carbonate nodules suggests possible affinities with the
megacryst suite. This interpretation is consistent with the occurrence of carbonate inclusions in ilmenite,
which has been documented as a late-crystallizing megacryst phase in the Monastery kimberlite. The mantlelike δ13C suggests a mantle origin for the nodules, which can be explained in both scenarios. However, the
textural evidence is very unusual for kimberlitic carbonates, which are typically in equilibrium with the
previously crystallised phases, and never show these quench textures. We tentatively suggest that the textural
evidence favours an interpretation for a mantle-derived origin for the carbonate rather than being related to
the crystallising kimberlitic assemblage. If this is indeed true, the presence of these coarse-grained carbonate
xenoliths/xenocrysts suggests that carbonate may be present in the mantle as coarse aggregates. Our
observations show that carbonate nodules occur in other kimberlites – for example the lower Palaeozoic
Colossus kimberlite on the Zimbabwe craton. They have possibly been overlooked in the past due to the high
abundance of late-stage near-surface carbonatization of many kimberlites.
33 WHERE DO WE LOOK? EYE TRACKING IN PETROLOGY
M.S. Huber1 F. Roelofse1 & D. Wium2
1
Department of Geology, University of The Free State, Bloemfontein, South Africa; [email protected]
2
Department of Computer Science and Informatics, University of the Free State, Bloemfontein, South Africa
The study of petrographic thin sections is critical to geological endeavours. However, the teaching
of petrology often presents challenges, as the specific details of importance are often difficult to articulate.
Therefore, we have attempted to approach this problem with eye tracking techniques. Experts at the 2016
IMSG meeting in Langebaan were asked to identify 30 petrographic images as igneous, sedimentary, or
metamorphic while viewing the images on eye tracking hardware.
Of the 32 participants, who ranged from 3 to 40 years of experience, 27 were able to correctly
identify at least 2/3 of the images correctly, providing a reasonable “expert” database to examine. The
experts, who were from the igneous and metamorphic study group, were prone to make errors assuming that
sedimentary samples were either igneous or metamorphic, with users selecting on average 7 images as
sedimentary, 12 as igneous, and 11 as metamorphic (when in reality, there were 10 of each). Years of
experience did not strongly correlate with the number of images correctly identified, although the majority of
users with the lowest scores had < 10 years of experience. Only one user was able to correctly identify all 30
images.
The experts had a tendency to look first at minerals when examining images. The interiors of the
minerals were examined, and once the major minerals have been identified, the gaze switches to examination
of grain boundaries. Often, experts are able to make an identification of rock type within 2 seconds of seeing
an image. The motion of the eye across a petrographic section tends to follow set pathways. These
pathways can be quantified through the use of rose diagrams, which clearly show preferential eye motions
along petrographic images. Eyes of experts tend to fixate on major minerals to make identifications. When
clear indicators are present, such as fossils, it allows for a very quick identification. However, with more
difficult samples, expert eyes progressively move from “large” or obvious minerals to less obvious or
smaller mineral grains and textures.
This technique is preliminary, and has limitations. However, these findings may help petrologists to
learn more about how they are functioning when looking through a microscope, and may also help in
geoscience education.
34 ORIGIN OF VARIOLITIC LAYERS IN THE 2230MA HEKPOORT LAVA FORMATION (SOUTH
AFRICA)
F. Humbert & M.A. Elburg
PPM Group, Geology Department, University of Johannesburg, PO Box 524, Auckland Park, 2006
Johannesburg, South Africa
The basaltic Hekpoort Formation belongs to the Pretoria group, upper part of the Transvaal Supergroup in
the Kaapvaal craton. This formation is located in the Transvaal basin, crops out over an area of ca. 100,000
km2, and has a thickness of up to 900m. Although the Hekpoort Formation mainly consists of subaerial lava
flows and some rare volcanoclastic deposits, some flows, which occur 40 kilometres northeast of
Potchefstroom, can be described as ‘variolitic’ (Fig. 1a). These lava flows contain spherical features,
showing a diameter ranging from 1 to more than 10 centimetres (Fig. 1b), which can be described as
variolites or megaspherulites. The variolites are mainly composed of acicular clinopyroxenes (CPX) with a
diameter of 1µm, radiating from the core of the variolites to the rim (Fig. 1c,d). The variolites are interpreted
as reflecting crystallisation during fast cooling and not liquid immiscibility or devitrification/alteration as
proposed for other variolitic volcanics (e.g., Hanksi, 1993). This interpretation is based on the variation of
several elements concentration between the variolites and the groundmass (Fig. 1b), either relatively to the
bulk rock composition concentrated inside the groundmass (Na, Eu, Cs, Rb, Ba, K, Sr) or inside the
variolites (Ca, Mg, Fe). Previous studies done on similar modern rocks (e.g., Gardner et al., 2014) explain
this diffusion by the crystallisation of the acicular CPX composing the variolites, with the elements that are
incompatible in the CPX being enriched in the groundmass. Such diffusion can only happen above the
temperature of glass transition. A peculiarity of the Hekpoort variolitic flows is that they also contain another
primary texture interpreted to reflect fast cooling, namely skeletal CPX (Fig. 1c). Those CPX have lengths of
up to 4 cm, do not show a shape preferred orientation, and are distributed evenly between the variolites and
the groundmass. Their composition is marked by high Al2O3 and TiO2 contents, increasing from 5 to 10
weight% and 0.6 to 1.5 weight % respectively from the core to rim of the CPX. Such high contents in those
two elements are uncommon for CPX and, together with their skeletal shape, support the interpretation of
fast cooling.
Figure 1: (a) variolitic lavas flow exhibiting scattered variolites. (b) polished section of one half of a
variolite, and its surrounding groundmass. The ‘enriched groundmass’ contains higher Na, Eu, Cs, Rb, Ba,
K, and Sr compared to the ‘regular’ groundmass. (c) detail of (b) under crossed polarizers showing acicular
CPX surrounding the skeletal CPX, here in basal cross section. (d) backscattered electron image of the
section highlighted in (c) showing the acicular CPX.
Gardner, J.E., Befus, K.S., Miller, N.R., Monecke, T., 2014. Cooling rates of mid-ocean ridge lava deduced
from clinopyroxene spherulites. Journal of Volcanology and Geothermal Research 282, 1–8.
Hanski, E.J., 1993. Globular ferropicritic rocks at Pechenga, Kola Peninsula (Russia) Liquid immiscibility
versus alteration. Lithos 29, 197-216.
35 FORMATION OF POTHOLES IN CONJUNCTION WITH SYN-MAGMATIC EXTENSION
E.J. Hunt1, R. Latypov1, M.S.D. Manzi1, P. Horváth1 & D. Hoffmann2
1
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;
[email protected].
2
Mineral Resource Department, Lonmin Platinum, South Africa.
Pothole structures in layered igneous complexes are where one of the horizons transgresses
downwards into the underlying stratigraphy. They are best recognised in the Merensky Reef (MR) of the
Bushveld Complex, where the potholes provide key insights into the processes through which the layering
and the critical metals it hosts (PGE, Cr and V) developed. Despite detailed studies on potholes over the last
~70 years there is very little consensus as to how they formed. Studies typically are based on geological
techniques including fieldwork and geochemical analyses, however these have not yet provided a widely
accepted model of the magma chamber processes that led to the development of potholes.
This study presents an analysis of pothole structures within the Karee region of the Bushveld
Complex (~30 km E of Rustenburg, ~75 km NW of Johannesburg), using a combination of traditional
geological techniques (field, petrographic, textural and mineral chemical) with geophysical (3D reflection
seismic) studies. This represents a novel approach to understanding magmatic processes and provides much
greater insight into the development of complex 3D magmatic structures than either method can provide
alone.
The general lack of exposure within the Bushveld Complex means that potholes are rarely exposed
and often only briefly within open pit mines. This led our study to use high-resolution 3D seismic data to
resolve potholes and other geological structures at an approximate scale of 10 m, which have direct impact
on future mine planning and designs. Data from the Lonmin Karee Mine indicate that potholes often occur in
conjunction with large-scale faults that traverse the Critical and Lower Zones to the basement rocks. Synmagmatic extension has previously been postulated to account for the development of potholes, based on
widespread evidence of deformational features throughout the Critical Zone, the morphology of potholes and
Sr R0 values that do not reflect large-scale contamination of the MR magma with footwall rocks [1, 2].
However little textural or outcrop evidence has previously been presented.
Our fieldwork found that sheets of fine-grained rocks (often called brown sugar norite or BSN within
the mines) are commonly associated with potholes, either within 5 m up-dip of a pothole structure or within
the pothole immediately (<2 m) before the reef dip abruptly changes (>30º). These rocks are fine-grained (<1
mm) and leuconoritic in composition (70% plagioclase, 27% orthopyroxene and 3% clinopyroxene) in
comparison to the typical medium- to coarse-grained (3-7 mm) melanoritic potholed MR (55%
orthopyroxene, 40% plagioclase, 3% clinopyroxene and 2% sulphides). Despite these mineralogical
differences, the fine-grained leuconorite is compositionally similar to the MR with orthopyroxene Mg#
(Mg/(Mg+Fe2+)) ratios between 0.81 and 0.83, while the potholed MR Mg# ratios range between 0.78 and
0.83, implying a genetic relationship. Textural analysis indicates that the fine-grained leuconorite is marked
by a strong fabric defined by mineral shape long axes (plagioclase and orthopyroxene) being orientated
parallel to the dip trend of the MR melanorite. The orientation of mineral grains and reduced grain sizes in
comparison to the MR are interpreted to reflect syn-magmatic shearing of a semi-consolidated crystal
framework during development of potholes. This provides further evidence of a structural control for the
initiation of potholes, with additional processes (most likely erosion with minor remobilisation of crystals)
acting to develop the preserved pothole morphologies.
[1] Carr et al. (1994). Economic Geology, 89, 1398-1410.
[2] Carr et al. (1999). Mineralium Deposita, 34, 335-347.
36 THE WATERBERG PROJECT – ATYPICAL BUSHVELD MAGMATISM AND
MINERALIZATION
F.M. Huthmann1, J.A. Kinnaird1, M.A. Yudovskaya1,2, M. McCreesh1,3, M. A. Elburg4, D. Frei5, T. Botha3
1
University of the Witwatersrand, Johannesburg, South Africa
IGEM, Moscow, Russia
3
Platinum Group Metals, Platinum House, 24 Sturdee Ave, Rosebank, South Africa
4
University of Johannesburg, Johannesburg, South Africa
5
University of Western Cape, Cape Town, South Africa
2
Despite the ongoing depressed mining environment, the platinum group metals platinum, palladium and
rhodium are an important commodity with crucial applications in the production of auto-catalysts, fuel cells
and other industrial applications. Ever since the discovery of the platinum resources of the Bushveld
Complex in the early 20th century and the subsequent exploration for more mineralization, the Northern
Lobe was assumed to end somewhat south of the Hout River Shear Zone (Fig. 1). The discovery of a worldclass ore body by Platinum Group Metals immediately north of, but not in the Bushveld is therefore perhaps
surprising. The latest results for both mineralized zones are 24.9 million ounces Pt+Pd+Rh+Au indicated and
12.3 million ounces 4E probable reserve (2.5 g/t cut-off).
The Waterberg Project is located north of the Northern Lobe of
the Bushveld Complex (e.g. Kinnaird et al., 2015) and just
north of the Hout River Shear Zone (Fig. 1). It contains a 2056
to 2059 Ma (Huthmann et al., 2016), >3.5 km x 24 km, lobate,
mafic-ultramafic succession of up to 1200 m thickness
unconformably overlain by up to 750 m of Paleoproterozoic
rocks of the Waterberg Group (Huthmann et al., 2016). The
succession is subdivided into an Ultramafic Sequence (UmS)
comprising harzburgite and feldspathic pyroxenite overlain by
what we refer to as the Troctolite-Gabbronorite-Anorthosite
Sequence (TGA Sequence) and in some areas Upper Zone
ferrogabbro/-gabbronorite. The UZ contains disseminated
magnetite and only one hole in the far south of the project has
intersected magnetitite. The igneous units are gently dipping
and increasing in thickness toward the the west-southwest.
We present data from a range of studies comprising core
logging, whole-rock chemistry and 6 element PGE data. This
is supplemented by newly-acquired Sr isotopic data for
samples covering the whole succession.
Given the mineralogical and and geochemical characteristics
of the Waterberg Succession, we argue it represents a separate
subchamber rather than a marginal extension of the Bushveld
Complex. Its emplacement history started with finger-like
chonolithic intrusions followed by lateral dilation and emplacement of sulfide droplet-bearing ultramafic
lithologies. This in turn was followed by a second phase of intrusives, forming a sheet-like body of
troctolites, later fractionating into gabbroic rocks. The position of both silicates and mineralization may be
controlled by its unique structural position north of the Hout River Shear Zone.
Figure 1: Geology of the Northern Lobe and location of the Waterberg Project.
[1] Kinnaird, J. A., Nex, P. A. M., 2015. An Overview of the Platreef. In: Platinum-group element (PGE)
mineralisation and resources of the Bushveld Complex, South Africa. pp. 193 342.
[2] Huthmann, F. M., Yudovskaya, M. A., Frei, D., Kinnaird, J. A., Jul. 2016. Geochronological evidence for
an extension of the Northern Lobe of the Bushveld Complex, Limpopo Province, South Africa. Precambrian
Research 280, 61 75.
37 THE RELATIONSHIP BETWEEN CARBONATITIC, MELILITIC AND POTASSIC TRACHYTIC
MAGMA TYPES AT THE SALTPETERKOP COMPLEX, SUTHERLAND, SOUTH AFRICA
P.E. Janney1 & M. Marageni1
1
Department of Geological Sciences, University of Cape Town, Rondebosch 7700 [email protected]
The Saltpeterkop carbonatite complex near Sutherland, South Africa, is unusual in that it is one of the few
southern African carbonatite localities with reasonably well-preserved volcanic features, including a 1 kmdiameter tuff ring composed of silicified volcaniclastic breccia. Around the complex, the regionally flatlying Karoo strata have been dramatically upwarped, with dips away from the Complex as high as 45° and a
pronounced radial fracture systems. Further, within about a 10 km radius of the central tuff ring are several
intrusive bodies of potassic trachyte and a large number of dikes, sills, and diatremes/plugs composed mainly
of carbonatite and olivine melilitite with the spatial density of these intrusions decreasing with increasing
distance fromt the centre. We have recently completed an in-depth geochemical reconnaissance of the
Saltpeterkop complex, involving field sampling and whole-rock major and trace element analysis, with
radiogenic and stable isotope measurements in progress. While the association with potassic trachytes is
relatively common in southern African carbonatites, the presence of significant amounts of primitive olivine
melilitite (30-40 wt.% SiO2, Mg# = 61-74) is unusual. Our preliminary model for the origin of the complex
involves the following stages:
(1) Ascent and intrusion of a mantle-derived carbonate-rich and mildly potassic magma (likely
ultramafic lamprophyre) into the lower-mid crust.
(2) Separation of an alkali carbonate phase from this magma, resulting in intensive fenitization and
partial melting of lower and/or mid-crustal rocks, the heat from the magmatic input and soliduslowering effect of the fenistisation causes a significant volume of the fenitised rocks to melt and
ascend into the upper crust, which likely explains much of the upwarping and fracturing of the
local Beaufort-group sandstone. The potassic trachyte is exposed at the surface mainly as
poorly exposed intrusive bodies within 2 km of the centre of the complex, often showing
significant stoping and brecciation of the country rock sandstone.
(3) The remaining (less potassic) primitive carbonated magma separates via liquid immiscibility or
possibly fractional crystallization into carbonatite (more abundant) and primitive olivine
melilitite magmas (less abundant) There are also rare carbonated ultramafic lamprophyres (Mg#
49 to 66) exposed, having up to 20% LOI, mainly due to high carbonate content.
(4) Both carbonatites and melilitites are emplaced mainly as dykes and sills and, more rarely, as
diatremes or subsidiary vents. The carbonatite spans a wide compositional spectrum,
apparently due to fractional crystallization, from calciocarbonatite (up to 45 wt. % CaO) to
ferrocarbonatite (up to 19 wt.% FeO and 4 wt.% MnO and up to 2 wt.% total rare rare earth
oxides), with some rare carbonatite types containing significant MgO (5-11 wt.%), as well as
CaO (19-30 wt.%), Fe2O3tot. (10-18 wt.%) and MnO (1-3 wt.%).
(5) Hydrothermal alteration is most pervasive and intense in the region inside and immediately
surrounding the tuff ring, and becomes less pervasive further from the ring structure. This
alteration does not appear to have had a significant effect on the REE contents of the affected
carbonatites (in many cases the carbonate removed appears to have been replaced by silica).
38 PETROLOGY AND GEOCHEMISTRY OF OLIVINE MELILITITE PIPES IN NAMAQUALAND
SOUTH AFRICA: NEW CONSTRAINTS FROM RADIOGENIC AND OXYGEN ISOTOPE DATA
M. D. Kirchner1, P. E. Janney1 & C. Harris1
1
Department of Geological Sciences, University of Cape Town, South Africa. [email protected]
Olivine melilitites are intermediate, in terms of degrees and depths of melting, between alkaline basalts and
kimberlites. The olivine melilitites in Namaqualand, South Africa form part of the NE trending, age
progressive Early Tertiary to Late Cretaceous “Namaqualand-Bushmanland-Ariemsvlei” (NBA) alkaline
igneous lineament, composed of hundreds of diatremes and pipe-like subvolcanic intrusions, which extends
from Namaqualand in western South Africa to southeastern Namibia and southern Botswana. Rock types in
the NBA lineament include ultramafic lamprophyres and kimberlites (northern Bushmanland and Ariamsvlei
cluster) as well as olivine melilitites (in Namaqualand and Bushmanland).
The present study aims to describe the petrogenesis of a cluster of 15 Palaeogene (≈55 Ma) subvolcanic
pipes in Namaqualand, near the west coast of South Africa, between the towns of Bitterfontein and Garies.
The Namaqualand olivine melilitites are particularly unusual because, although they were emplaced as
diatremes and show evidence for emplacement driven by exsolution of volatiles similar to kimberlites, they
are variably differentiated. Petrographically, this suite of rocks shows significant variation in modal
composition and textures within individual pipes and especially between different pipes. The samples
generally contain olivine phenocrysts which vary in modal abundance between ~2 and ~30 %,
titanomagnetite and ilmenite phenocrysts and microphenocrysts (up to 15 %), and tabular melilite
microphenocrysts (up to ~20 %). Clinopyroxene (up to ~40%), perovskite (~1 to 3%), and nepheline (0 to
~15 %) tend to dominate the groundmass. Generally, the samples obtained are very fresh. Due to their
freshness, the whole rock geochemistry is indicative of the original magma composition after degassing.
Mg numbers (atomic Mg/[Mg + Fe2+]*100)for igneous rocks from the Namaqualand pipes range from 71 to
49, with most having values below 60. In contrast, melilitites from the Bushmanland cluster roughly 100 km
to the northeast, are near-primary, with Mg numbers between 70 and 80 (similar to kimberlites). The wide
variation in Mg numbers for these magmas is most likely mainly due to olivine crystal fractionation prior to
or during eruption. It is predicted that between 6 and 60 % olivine fractionation would be required to explain
these compositions.
Radiogenic Sr, Pb and Nd isotope compositions of whole rock samples from the Namaqualand cluster of
pipes overlap with those for the Bushmanland pipes. The Namaqualand pipes however show higher initial
206
Pb/204Pb values (over 21.0 in some cases) than the Bushmanland data, suggesting some differences in
source composition or the influence of crustal assimilation. The Namaqualand and Bushmanland melilitite
pipes also have whole rock radiogenic isotope compositions that overlap with South African Group 1
kimberlites, but extend to higher initial 206Pb/204Pb values and have somewhat different Nd-Hf characteristics
suggesting a range of mantle sources with similar, but not precisely the same composition.
Trace element data (e.g., Ce/Pb ratios) show no evidence for crustal assimilation and the Namaqualand pipes,
as a group, have relatively uniform incompatible element ratios. However, olivines from nine pipes span a
range of δ18O values between +4.22 and+ 5.22‰. The upper part of this range is consistent with derivation
from normal mantle peridotite, but the lower end is outside the normal range for olivine in mantle-derived
magmas. It is pertinent to note that the two pipes from which olivine δ18O values of less than +4.9‰ were
obtained are both located within the Early Cretaceous Koegelfontein Complex, a felsic intrusive complex
that is associated with unusually low δ18O values. These anomalously low δ18O values are, therefore,
consistent with some cryptic crustal assimilation.
39 THOLEIITIC TO CALC-ALKALINE METAVOLCANIC TRANSITION IN THE ARCHAEAN
NIGERLIKASIK SUPRACRUSTAL BELT, SW GREENLAND
M.B. Klausen1 & K.U. Szilas2 & T.F. Kokfelt3
1
Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa;
[email protected]
2
Department of Earth and Environmental Sciences, Stanford University, 94305 CA, USA.
3
Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland (GEUS),
1350 Copenhagen, Denmark.
We present bulk rock geochemical and U-Pb zircon age constraints on a logged ~550 m thick sequence of
Mesoarchaean metavolcanic rocks from the Nigerlikasik peninsula of SW Greenland. The sequence includes
picritic serpentinites at the base (~30 m), succeeded by tholeiitic basaltic amphibolites (~220 m) and calcalkaline andesitic-dacitic schists at the top (~300 m). All rocks were deformed into a steep flank of a large
synform and metamorphosed under amphibolite facies conditions, but relict volcanic structures clearly testify
to their igneous origins, including fiamme-textured ignimbrites, pyroclastic breccia-flows and rare pillowbasalts. Underlying parts of the lava sequence were arguably intruded by syn-volcanic gabbroic and felsic
feeder intrusions. The sequence was also intruded by late aplite sheets, particularly in the lower part, of
which four were U-Pb dated, producing a minimum age for the entire metavolcanic succession of 2931 ± 4
Ma. The surrounding regional grey orthogneisses post-date the metavolcanic sequence and a sample near the
base yields a protolith intrusion age of 2903 ± 4 Ma. Up through the metavolcanic section, there is an overall
increase in SiO2 (and other more incompatible elements) and coupled decreases in CaO, MgO (and other
more compatible elements), but with a major change, nearly halfway, from a distinctly tholeiitic meta-basalt
to calc-alkaline meta-andesites to dacite suite. Each suite exhibits cyclic variations that are most consistent
with magma chamber replenishments and the fractionations of tholeiitic gabbro and a calc-alkaline
amphibole ±garnet assemblages. Distinctly different geochemical signatures, argue for each suite having
been derived from two distinct source regions: (1) the tholeiitic metabasalts have high-FeOT and lowAl2O3/TiO2 with flat REE-patterns and are interpreted as being mantle-derived; (2) the evolved calc-alkaline
metavolcanics have relatively low-FeOT and high-Al2O3/TiO2 with steep HREE-depleted patterns, consistent
with a garnet amphibolite crustal source. Additional gradual (Dy/Yb)N decreases up through the lowermost
picrites and basalts are explained in a context of decreasing degrees and depths of mantle melting, whereas a
subsequent transition into relatively high- and low (La/Yb)N tholeiites argue for a correspondingly more
depleted mantle source. It is more difficult to explain the subsequent emplacement of such a radically
different andesitic calc-alkaline suite, with relatively high MgO, Ni and Cr, even when considering a number
of different plate tectonic scenarios. It is also difficult to explain the variations of every element within the
andesite group simply through mixing of a mantle-derived tholeiitic and a crustal-derived aplitic endmember, as proposed for many modern island arc setting. Instead, we hypothesize that the large throughput
of tholeiitic magmas, below a volcanic centre and above a relatively hotter Archaean mantle, generated
primary andesitic melts through >50% partial melting of the garnet amphibolite base of a thickened island
arc. It is further speculated upon whether these melts collected into a differentiating deep magma chamber,
into which higher density tholeiitic magmas were trapped, mingled and heated the system. The volcanic
system eventually terminated together with Nigerlikasik’s last record of rhyodacitic intrusions.
40 HOW LITHOLOGY AFFECTS THE DEVELOPMENT OF PSEUDOTACHYLITIC BRECCIA
DURING LARGE IMPACT EVENTS
E. Kovaleva & M.S. Huber
Department of Geology, University of the Free State, 205 Nelson Mandela Drive, 9300 Bloemfontein, South
Africa; [email protected]
Pseudotachylitic breccia (PTB) is abundant in the core of the Vredefort impact structure, and has been found
in all pre-impact lithologies, including metaigneous [1, 2, 3] and metapelitic [4] rocks. However, the
mechanisms of how this melt rock type forms remain highly debated. Various authors have suggested: a
combination of brecciation, friction and shock melting during the compression stage of the crater formation
[5, 6]; reduction in pore fluid pressure [7] and shock-induced collapse of pre-existing fracture-related voids
[8, 9, 10]; post-shock decompression melting [6]; frictional heating during the late-stage crater development
and modification [3, 4, 11, 12]. Structural inhomogeneities, such as lithological contacts or faults, can cause
locally elevated shock conditions and a combination of shearing/faulting to produce shock melting [9, 10],
and thus often provide nucleation planes for PTB melt [1]. We suggest that the principal mechanisms of PTB
formation vary depending on the target rock type.
In the core of the Vredefort impact structure, meta-gabbro and meta-granite are observed in contact with
each other. A set of PTB veins cuts through both lithologies. Granitic clasts are found within PTB veins in
meta-gabbro, close to the contact with granite. This indicates that melt in both rock types was active and
mobile during the same period of time, with physical mixing and chemical exchange between the melts.
Thus, PTB cuts across the contact between granite and gabbro, and not along their contact, as suggested in
[1, 9, 10].
Detailed microstructural analyses of the PTB veins in thin sections have revealed differences between PTB
developed in meta-granite and in meta-gabbro. In granitic samples, PTB often develops along physical
contacts, such as a contact with a migmatite or pegmatite vein. Nucleation sites of PTB have features
suggesting ductile deformation and shearing. We observe preferential melting of mafic and hydrous
minerals, in agreement with [1, 6]. Refractory phases remain in the melt as clasts and form reaction rims. In
contrast, PTB in meta-gabbro is preceded by brittle deformation of the host rock, and do not exploit existing
physical contacts. Cataclastic zones develop along the faults and progressively produce ultracataclasites and
melt. Thus, PTB veins in meta-gabbro contain fewer clasts that usually represent multi-phase fragments of
host rock and not specific phases. Such fragments often originate from the material trapped between two
close parallel-running faults.
These differences in nucleation and propagation mechanisms of PTB melts based on rock type must be
considered when discussing the formation mechanisms of impact-generated PTB.
References:
[1] Reimold & Colliston (1994) Geol Soc Am Spec Papers 293:177-196. [2] Mohr-Westheide et al. (2009) S
Afr J Geol 112:1-22. [3] Mohr-Westheide & Reimold (2010) Geol Soc Am Spec Papers 465:619–643. [4]
Gibson et al. (1997) Tectonophysics 283:241-262. [5] Dressler & Reimold (2004) Earth-Sci Rev 67:1-54. [6]
Reimold et al. (2015) Proceedings, 46th LPS Conference:1035. [7] Killick & Reimold (1990) S Afr J Geol
93:350-365. [8] Martini et al. (1991) Earth Planet Sc Lett 103:285–300. [9] Kenkmann et al. (2000) Meteorit
Planet Sci 35:1275-1290. [10] Langenhorst et al. (2002) Meteorit Planet Sci 37:1541-1553. [11] Spray
(1995) Geology 23:1119–1122. [12] Spray (1997) Geology 25:579–582.
41 HYPATIA REVISITED: THE COOLEST STONE
Kramers, J.D.1, Belyanin, G.A.1, Greco, F.1 & Andreoli, M.A.G.2
1
Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa;
[email protected]
2
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
The ca. 30 g stone named “Hypatia” found in 1996 in the Libyan Desert Glass area of Southwest Egypt was
first shown to be of extraterrestrial origin by [1] and in view of its unique carbon-dominated chemistry, a
cometary origin was suggested1. A follow-up noble gas and nitrogen isotope study2 characterized noble gases
of the “Q” type common in the organic matter of chondritic meteorites and found that Hypatia was part of a
large bolide, but concluded also that this would probably have been a differentiated solar system object
rather than a comet. On the other hand, element distribution patterns obtained by Proton Induced X-ray
Emission (PIXE) analysis3 could not be explained by known differentiation processes. We conducted a
mineral-chemical study (EMPA-WDS and SEM-EDS) on non-carbon minerals and Raman spectroscopy on
the carbonaceous matrix to address the questions of shock metamorphism and pre-shock thermal processing
of the object.
Two distinct types of matrix can be clearly distinguished: Type 1 consists of almost pure carbon (of elements
that can be analysed by WDS or EDS, i.e. not considering N or H), while Type 2 contains up to 5% of a mix
of mainly Fe, S, Ni and P as well as some O. Cloudy concentrations of sub-µm grains of FeS (pyrrhotite or
troilite) characterize Type 2 but its overall Fe/S atomic ratio > 1 indicates at least Fe is partially hosted in the
organic matrix. Clusters of <0.2 µm diamonds occur exclusively in Type 2. Grain sizes of these minerals and
lamellae in the diamonds are typical of shock-induced crystallization4,5. Among probably presolar minerals
seen in Type 2 are SiC (up to 5 µm) and a Ni-phosphide phase (in aggregates up to 60 µm) with variable
atomic Ni/P > 5, a phase not observed before. A strong chemical disequilibrium exists between this phase
(Ni/Fe >30) and the matrix in which it is embedded (Ni/Fe < 0.1).
Raman spectra of matrix type 1 are rather uniform and show the carbon D (“disorder”) and G (“graphite”)
bands at Raman shifts of c. 1350 and c. 1600 cm-1, respectively, superimposed on a background of strong
fluorescence. The bands’ intensity ratio ID/IG ranges from 0.54 to 0.9, values typical of very primitive (not
thermally processed) organic matter in carbonaceous chondrites and interplanetary dust particles (IDP’s)6 but
full width at half maximum (FWHM) of the D band is lower at 80-150 cm-1, more typical of moderately
processed CV and ordinary (O) chondrites7,8. A mismatch also exists for the G-band, which has FWHM
values of 90-140 cm-1, similar to primitive matter (including comet Wild2 stardust9) , but a centre Raman
shift range of 1597-1607 cm-1, close to CV and O chondritic values. Matrix type 2 Raman spectra reveal the
presence of diamond by very narrow and prominent D bands (FWHMD 30-40 cm-1 and ID/IG 1.2-4.5) while
the G band changes to lower centre shift (down of 1583 cm-1) and greater FWHM (up to 155 cm-1) values as
it decreases in intensity.
We argue that these apparently contradictory features of the carbonaceous matrix and the non-carbon
mineralogy and mineral chemistry are in accord with a scenario in which a very primitive (i.e. least thermally
processed) object underwent shock metamorphism at pressures high enough to generate diamond. An impact
is not required for this as a high velocity entry into the atmosphere can generate such shock metamorphism10.
The primitive nature of the pre-entry bolide can explain the non-Goldschmidt chemistry3 of the object and
motivates further chemical and isotope studies on Hypatia.
1
Kramers et al. 2013. EPSL 382, 21-31. 2Avice et al., 2015. EPSL 432, 243–253. 3Andreoli et al, 2015. NIMB, 363, 7985.; 4 Koeberl et al., 1997. Geology 25, 867–970. 5Tschauner et al., 2015. Science 346, 1100-1102. 6Quirico et al.,
2006. Planet Space Sci 53, 1443-1448. 7Quirico et al., 2003. Met Planet Sci 38, 795-811. 8Bonal et al., 2006. GCA 70,
1849-1863. 9Sandford et al., 2006. Science 314, 1720-1724. 10Boslough & Crawford, 2008. Int J Impact Eng 35, 14411448.
42 THE SUDBURY IGNEOUS COMPLEX HAD THE UPPER BORDER SERIES!
R.M. Latypov1, S.Yu. Chistyakova1, V.V. Egorova2, H. Huhma3 & A. Brown1
1
School of Geosciences, University of Witwatersrand, Johannesburg, South Africa; [email protected]
2
Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
3
Geological Survey of Finland, Espoo, Finland
The Sudbury Igneous Complex (SIC) has formed from a melt sheet generated in a matter of minutes by a
large asteroid impact. Subsequent closed-system cooling and crystallization of the melt sheet is thought to
have produced a sequence of norite, quartz gabbro, and granophyre from base to top. The recent discovery of
several abrupt compositional reversals within norite and quartz gabbro sequences about one to two km above
the base of the complex suggests, however, that this simple story of intrachamber evolution of the SIC must
be questioned. The compositional reversals are associated with ~10 to 350 m thick sulphide-bearing
melanocratic norite bodies (MNB) which are similar to the Sublayer mafic norites at the base of the SIC in
major element composition and in occurrence of disseminated sulphides. The Sublayer norites are, however,
different from them in that the MNB have both orthopyroxene and plagioclase as cumulus phases, contain a
lesser amount of interstitial material, and show a well-developed plagioclase lamination. The MNB are
characterized by elevated orthopyroxene content, whole-rock MgO abundance and An-content of
plagioclase. Notably, the MNB are indistinguishable from host norite and quartz gabbro in incompatible
element concentration ratios (e.g. La/Y) and in whole-rock Nd isotopic composition. In particular, the initial
εNd in the MNB (-8.0) are within error of those in all mafic rocks of the SIC. A geochemical affinity of all
noritic rocks of the SIC is also illustrated by similar primitive mantle-normalized trace element concentration
patterns that show prominent Nb, Ta, P and Ti negative anomalies. Three explanations for the origin of the
MNB can be envisaged in the context of a closed-system model for the SIC: (1) a fault-induced structural
repetition of weakly mineralized Sublayer norites; (2) a reversal in crystallization sequence in response to a
sudden change in lithostatic/fluid pressure in the magma chamber; (3) a lateral displacement of resident
magma from neighboring sub-chambers of the SIC. None of these explanations appears to be adequate to
explain certain features of the MNB. As an alternative, we propose here that these bodies are autholiths of
the initially existed rock sequence that crystallized from roof downwards (Upper Border Series) but was later
disrupted by tectonics and collapsed on the chamber floor. If proven true, this novel idea will require a
substantial re-evaluation of current models for internal crystallization and differentiation of impact melt and
possibly for origin of Cu-Ni-PGE sulphide ores of the SIC.
43 DETACHMENT FOLDING OF PARTIALLY MOLTEN CRUST IN ACCRETIONARY OROGENS:
A NEW MAGMA-ENHANCED VERTICAL MASS AND HEAT TRANSFER MECHANISM
J. Lehmann1, K. Schulmann2,3, O. Lexa2, P. Závada2,4, P. Štípská2,3, P. Hasalová2,4, G. Belyanin1 & M.
Corsini5
1
Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
2
Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, Prague 1, 11821, Czech Republic
3
Ecole et Observatoire des Sciences de la Terre, Institut de Physique du Globe de Strasbourg – CNRS
UMR7516, Université de Strasbourg, 1 rue Blessig, F-67084, Strasbourg Cedex, France
4
Institute of Geophysics ASCR, v.v.i., Boční II/1401, Prague 4, 14131, Czech Republic
5
Géoazur, UMR 7329, CNRS, University of Nice - Sophia-Antipolis, 250 rue Albert Einstein, Valbonne
Cedex, France
We use structural, petrographic and geochronological data to examine processes of exhumation of partially
molten crust in the late Devonian–early Carboniferous Chandman dome in the Mongolian tract of the Central
Asian Orogenic Belt. The dome is made of a high-grade core of granitoids and migmatites and a low-grade
metamorphic envelope. Its tectonic evolution can be divided into three main stages. The oldest fabric is a
sub-horizontal foliation S1 in migmatites that is sub-parallel to magmatic foliation in granitoids and to
greenschist facies schistosity in the enveloping metasediments. This event is interpreted as a result of
horizontal deep crustal flow at depth ~ 20 – 25 km. The S1 layering was subsequently transposed into a new
foliation S2 or affected by open to close upright F2 folds that are locally truncated by steep walls of
diatexites suggesting influx of partially molten crust into fold cores. The shallow-dipping magmatic foliation
in granitoids is locally reworked by vertical magmatic to gneissic S2 fabrics. Syn-S2 metamorphic
assemblages and syn- to post-kinematic cordierite point to magma-accommodated upward entrainment of the
high-grade core, coeval with isobaric heating of the surrounding upper crust at ~ 10 km. Steep and S2parallel leucogranite sheets cross-cutting both magmatic core and mantling migmatites either exploit S2 or
cross-cut horizontal S1 fabrics. Together with new 40Ar/39Ar ages that overlap with previously published
crystallization ages, this suggests that the dome formation terminated by the brittle expulsion of magma
along S2 vertical dykes during continuous cooling. We suggest that this tectonic evolution can be explained
by detachment folding and vertical exhumation of partially molten lower crust parallel to axial fold plane
coevally with erosional unroofing of upper crust above the hinge of antiform. In order to justify this
mechanism we apply analogue model with temperature dependent rheology of the lower crust represented by
a partially molten wax layer. It is shown that the fold core initially filled by under-pressurized and buoyant
magma becomes rapidly over-pressurized during fold lock-up enhancing upward extrusion of magma and
migmatites. The continuous lateral shortening triggers further vertical extrusion of buoyant magma along
vertical dykes. In our view, detachment folding model explains well the lateral alternation of low-grade
domains and migmatite-magmatite zones commonly observed in accretionary orogens worldwide.
44 GEOCHEMICAL CHARACTER OF THE VLOCÁN AZUFRE, CHILE
J.J.S. Lister, P. Le Roux & C. Harris
Department of Geology, University of Cape Town, Cape Town, South Africa; [email protected]
This study presents Sr-, Nd-, and O- isotope data from andesitic and dacitic lava flows that were erupted
from the Volcán Azufre in the Andean Central Volcanic Zone (CVZ). Azufre is at the western end of a
cross-arc volcano chain, similar to the nearby San Pedro – Linzor chain which shows age-progression
younging to the west. The chain of which Azufre is a part of is relatively unstudied when compared to the
San Pedro – Linzor chain, and this provides an opportunity for further study and assessment of Andean
magmatic evolution and possible crustal assimilation. The 87Sr/86Sr and 143Nd/144Nd isotope ratios were
determined via MC-ICP-MS mass spectrometry and δ18O values by laser fluorination. The SiO2 content of
the samples ranged from 59.17 to 65.66 wt.%; corresponding to 10 andesites and 2 dacites. The Azufre lavas
have 87Sr/86Sr ratios that vary between 0.706680-0.707460. The δ18O values of quartz from samples JL-AZU002 and JL-AZU-004 are 8.13‰ and 9.37‰, respectively. These relatively high values for both 87Sr/86Sr and
δ18O are consistent with the addition of crustal material to the magma prior to eruption.
45 PALAEOMAGNETISM OF THE NSUZE GROUP OF THE PONGOLA SUPERGROUP,
KAAPVAAL CRATON
C. Luskin1,*, M.O. de Kock1, H. Wabo1, A.P. Gumsley2
1
Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg,
Gauteng, South Africa
2
Department of Geology, Lund University, Sölvegatan 12, 22362, Lund, Skåne, Sweden
* Corresponding author e-mail: [email protected]
The Mesoarchaean Kaapvaal Craton suffers from a lack of reliable, spatiotemporally dense paleomagnetic
data. This hampers our understanding of the early plate tectonic history of the Kaapvaal Craton and its
relationship to other Archean cratons. The Pongola Supergroup, located in the southeastern portion of the
Kaapvaal Craton, is inferred to cover 32,500 km2 and bears relatively undeformed and unaltered volcanic
sequences dating from ~2.99 to 2.87 Ga, making it an uncommonly ideal subject for Archean paleomagnetic
study. The lower Nsuze Group (~2.99-2.96 Ga) is rich in basaltic, basalt-andesitic, and rhyolitic lavas which
are relatively undeformed and generally of low greenschist metamorphic grade. The upper Mozaan Group
(~2.97-2.87 Ga) is primarily sedimentary, but bears two significant volcanic units.
There is debate over whether Pongola volcanics represent Archaean large igneous provinces (LIPs). Ernst
(2014) and Wilson & Hofmann (2013) refer to Nsuze volcanics as a LIP. Gumsley et al. (2013) propose
Mozaan volcanics along with the Hlagothi Intrusive Complex comprise a LIP. According to Ernst (2014), a
LIP requires a geographic extent > 100,000 km2, emplaced “in a short duration pulse or multiple pulses (less
than 1-5 Ma) with a maximum duration of < c. 50 Ma.” Accordingly, it may be premature to call Pongola
volcanics a “LIP,” as the original Pongola Basin was < 90,000 km2, and the timing of emplacement is not
resolved. If Nsuze Lavas, Dominion lavas, the Usushwana Complex, and the Badplaas Dyke Swarm
comprise a single province, their size would likely qualify as a LIP. Since “Archean LIPs represent
fragments of originally much larger LIPs, and their original scale and distribution will only become clear
once the late-Archean global reconstruction history has been sorted” (Ernst, 2014), paleomagnetic
investigation could help resolve these questions by establishing links between Pongola volcanics and other
coeval provinces of igneous origin (e.g., Ernst, 2014, p. 166).
This project aims to produce a large volume of high quality paleomagnetic data across the entire Pongola
Supergroup, focusing on Nsuze Group lavas. 147 paleomagnetic samples have been collected from Nsuze
lavas and related rocks of the White Mfolozi Inlier, and will be analysed at the University of Johannesburg
Paleomagnetic Laboratory. Previously collected samples from the Usushwana Intrusive Suite, Pongola
diamictites, a post-Mozaan quartz porphyry, and other localities will also be analysed. Additional samples
from Nsuze and Mozaan volcanics will be collected from the Buffalo River Inlier, Amsterdam Area,
Hartland Area, and in Swaziland to provide good spatiotemporal coverage of the Pongola Supergroup.
Using the collected data, this project will calculate paleopoles and paleolatitudes of the Kaapvaal Craton
during the period of Pongola deposition. Combined with geochronological data, results could yield an
apparent polar wander path for the Mesoarchaean Kaapvaal Craton and test the “Vaalbara” hypothesis. (de
Kock et al., 2009) Results may also shed light on the depositional environments of the Pongola Supergroup,
e.g., indicating whether paleolatitudes are consistent with a glaciated environment for the Klipwal diamictite
(Mozaan Group) and/or with a stromatolite-supporting tidal environment for the White Mfolozi Formation
(Nsuze Group). These findings could also bear upon paleolatitudes and depositional environments for the
correlated Dominion Group and Witwatersrand Supergroup. (Beukes & Cairncross, 1991) This project could
also test hypotheses that the layered Usushwana Complex is a coeval feeder of Nsuze lavas, and that the
mafic-ultramafic Hlagothi Complex fed Mozaan volcanics. (Gumsley et al. 2013, 2015) The data could help
evaluate whether these igneous events indicate a single mantle plume under a stationary craton, or shortlived plumes operating within a more active Mesoarchaean mantle driving plate movement.
Selected Reference List: Beukes & Cairncross, 1991, SAJG 94 (1): 44-69; Ernst, Richard, 2014, Large
Igneous Provinces, Cambridge Univ. Press; de Kock et al., 2009, Prec. Res. 174: 145-154; Gumsley et al.,
2013, Lithos 174 (1): 333-348; Gumsley et al., 2015, Prec. Res., 267: 174-185; Wilson et al., 2013, SAJG
116:119-168; Wilson & Hofmann, “June 2013 LIP of the Month,” www.largeigneousprovinces.org/13jun.
46 CONSTRAINING THE AMOUNT OF DECOMPRESSION AND THE P-T CONDITIONS IN HIGH
TEMPERATURE GRANULITE FROM BANDELIERKOP FORMATION (SOUTHERN
MARGINAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA)
N. Madlakana1 & G. Stevens1
1
Centre for Crustal Petrology, Department of Earth Sciences, Stellenbosch University, South Africa.
[email protected]
This study has investigated metamorphic reaction textures within restitic granulite facies metapelites from
the South Marginal Zone of the Limpopo belt. The rocks are characterized by spectacular cordierite and
orthopyroxene symplectites, which form pseudomorphs after garnet and which may have resulted from
decompression or compositional change, on melt extraction, or both. Past studies have not considered the
effect of bulk compositional change in producing these textures and proposed that the symplectites were
produced by decompression under granulite facies conditions. A recent past study by Nicoli et al (2014) has
applied a conventional phase equilibrium modelling approach to a range bulk composition from the same
outcrop to accurately constrain the possible conditions of decompression from 11 to 7 kbar at a temperature
of approximately 850 °C. This study aims to combine phase equilibrium modelling of restitic metapelites in
which both garnet and symplectite are preserved, with the results of high-resolution x-ray computer
tomography of a small volume cores in an effort to constrain the possible role of compositional change in
producing the textures and to quantify more precisely the metamorphic conditions recorded by the reaction
texture. A single core has been studied to date; garnet makes up 12.7 vol% of the rock, whereas the mode of
orthopyroxene and cordierite represent 25.7 and 32.2 vol % respectively. The following simplified balanced
reaction may be a reasonable approximation of the reaction, which has produced the cordierite +
orthopyroxene symplectites around garnet: 2Garnet + 3Quartz = Cordierite + 4Orthopyroxene. Using the
densities of the Fe-end-members as a proxy for the density of the solid solution phases involved, indicates
that the reaction produced a 15% increase in volume. Cordierite occurs exclusively within the reaction
texture. Thus, the volume of cordierite in the scanned volume (32.2%) may be used to calculate that 34.8%
of the core consisted of garnet that was consumed by the reaction, and that prior to the reaction the rock
contained 47.6% garnet. Similarly, this information can be used to calculate that 6.4% of the volume
represents orthopyroxene that was not produced by garnet breakdown. This information will be combined
with the results of phase equilibrium modelling to explore the details of the metamorphic process.
References
Nicoli, G., Stevens, G., Moyen, J-F., Frei, D., 2014. Rapid evolution from sediment to anatectic granulite in an Archean
continental collision zone: The example of the Bandelierkop Formation metapelites, South Marginal Zone, Limpopo
Belt, South Africa. Journal of Metamorphic Geology, doi:10.1111/jmg.12116.
47 ISOTOPIC RESETTING OF ZIRCON: INFLUENCE OF AGE, TEMPERATURE AND
CHEMICAL ENVIRONMENT, PRELIMINARY RESULTS
B.N. Magwaza1 & M.A. Elburg2
Department of Geology, University of Johannesburg, Auckland Park, South Africa;
[email protected]
Zircon grains are said to maintain excellent physical and chemical stability during high temperature events
despite long metamorphic history and that makes them extremely useful for geochronological purposes,
however, recent studies have postulated that the zircon dating may yield geologically meaningless age data
when the U-Pb and/or Lu-Hf isotopic systems in zircons are partially or totally disturbed depending on age,
temperature and the influence of chemical environment (Johnston et al., 2008; Kramers, 2009), and less
attention has been centered to zircon resetting in the past. The evidence of such disturbances may or may not
be observed on the images obtained from the techniques traditionally used to image zircons prior to analysis.
The current study aims to determine whether the isotopic systems in zircon may be disturbed by thermal
events such as the intrusion of the Bushveld Complex, and to what extent. The main in-situ micro analytical
technique used in this study is laser ablation multicollector-inductively coupled plasma mass spectrometer
(LA-MC-ICPMS), and imaging was done using cathodoluminescence (CL) technique. At this stage, three
samples have been analyzed. Most zircon grains in two samples of Archaean granitoids taken close to the
northern limb of the Bushveld Complex occur as inclusions in biotite and exhibit a variety of habits from
massive rounded to elongate prismatic, and are generally highly fractured and contain some inclusions. The
CL images are interpreted to reflect magmatic zircons (with well-defined oscillatory zoning), lobate zoning,
and some complex CL patterns. In one Magaliesberg Sandstone sample taken near the Schwerin fold, the
small rounded zircon grains occur as inclusions in quartz, and 90% of zircon grains are characterized by
overgrowths and complex CL patterns. For LA-MC-ICPMS U-Pb analysis ~99 zircon grains per sample
were analyzed using a 25µm spot size. Granitoid ME-MG-6 produced an upper intercept of 2805±32 Ma and
lower intercept of 420±140 Ma on a Wetherill concordia diagram with MSWD of 23, whereas granitoid
MEMG8 produced an upper intercept of 2808±39 Ma and lower intercept of 725±94 Ma with quite a high
MSWD of 30. The concordia diagrams for these two granitoid rocks show a similar linear trends with welldefined upper intercept ages, however, these ages do not reflect a true event at the lower intercept, and this
might be an indication of a post-crystallization event that resulted to dominant lead loss. The majority of
detrital zircons in the sandstone MEMG4 plot between ~2000 and 2500 Ma with a few discordant grains.
The Bushveld intrusive event might have affected the Magaliesberg Formation because the depositional age
of these rocks is ca. 2200 Ma (Mapeo et al., 2006; Dorland et al., 2004), but there is quite a high number of
zircon grains plotting at ca. 2055 Ma (BC age; Zeh et al., 2015) on the Wetherill Concordia diagram.
References:
Dorland, H.C., Beukes, N.J., Gutzmer, J., Evans, D.A.D., and Armstrong, R.A. (2004). Trends in detrital
zircon provenance from NeoarchaeanPalaeoproterozoic sedimentary successions on the Kaapvaal
craton: Abstract volume, Geoscience Africa 2004 Congress, Geological Society of South Africa,
Johannesburg, pp176-177.
Johnston, S., Gehrels, G., Valencia, V., and Ruiz, J. (2008). Small-volume U–Pb zircon geochronology by
laser ablation-multicollector-ICP-MS. Chemical Geology (2008), pp1-12.
Kramers, J., Frei, R., Newville, M., Kober, B., and Villa, I. (2009). The valency state of radiogenic lead in
zircon and its consequences. Chemical Geology 261, pp4-11.
Mapeo, R.B.M., Armstrong, R.A., Kampunzu, A.B., Modisi, M.P., Ramokate, L.V., and Modie, B.N.J.
(2006). A ca. 200 Ma hiatus between the Lower and Upper Transvaal Groups of southern Africa:
SHRIMP U-Pb detrital zircon evidence from the Segwagwa Group, Botswana: Implications for
Paleoproterozoic glaciations: Earth and Planetary Science Letters 244, pp113-132.
Zeh, A., Ovtcharova, M, Wilson, A.H., and Schaltegger, U. (2015). The Bushveld Complex was emplaced
and cooled in less than one million years – results of zirconology, and geotectonic implications. Earth
and Planetary Science Letters 418 (2015), pp 103-114.
48 PALAEOCENE-EOCENE MAGMATISM RELATED TO NEO-TETHYS CLOSURE, ALBORZ
MOUNTAINS, IRAN
S. Master1,2, J. Madonsela2, J.D. Kramers3, G. Belyanin3, R. Bolhar2, Shirazi, R.4
1
EGRI, 2School of Geosciences, University of the Witwatersrand, P. Bag 3, WITS 2050, Johannesburg,
South Africa, [email protected]
3
Department of Geology, University of Johannesburg, Auckland Park, Johannesburg, South Africa.
4
Ara Kooh Co., Tehran, Iran.
The Urumieh-Dokhtar Magmatic Arc (UDMA) of central Iran was formed since the early Cretaceous by
subduction related to the closing of the Neotethys Ocean, culminating in the late Neogene collision between
the Arabian and Iranian plates along the Bitlis-Zagros suture, and the formation of the Zagros fold belt on the
upper (Arabian) plate1. Abundant magmatism occurred in the UDMA during the Eocene and Miocene1.
Some Cenozoic magmatism related to this subduction event is also manifested in the Alborz Mountains
south of the Caspian Sea in North Iran2-7. The earliest magmatic event in the Alborz is the Akapol granite
batholith, with ages spanning the Palaeocene-Eocene boundary2. Other dated events include the c. 37.2 Ma
Mobarak Abad gabbros7 and Karaj Dam sill4, and the 6.8 Ma Alam Kuh Granite2.
We report on new 40Ar-39Ar ages of biotite monzodiorites from two phases of the Akapol granitoid batholith,
showing cross-cutting field relationships, from the Makaroud Valley in the vicinity of Kelardasht, in the
Alborz Mountains of northern Iran. The oldest phase is a coarse-grained biotite monzodiorite, which is cut
by fine-grained dark green mafic dykes, which in turn are cut by thin quartz diorite dykes. The biotite
monzodiorite AM1-4 has an excellent plateau age of 57.98 ± 0.31 Ma (latest Palaeocene, Thanetian)(Fig. 1).
This age is much more precise than the U-Pb age of 58 ± 3 Ma2, which has large errors because of low U,
and therefore low radiogenic Pb, contents. The quartz monzodiorite dyke is dated at 53.9 ± 2.4 Ma (earliest
Eocene, Ypresian) (Fig. 2). Our data overlap with the ages of magmatism in the Akapol batholith obtained
from U-Pb zircon dating (58 ± 3 Ma to 54 ± 4 Ma) and 40Ar-39Ar dating (56.8 ± 0.1 Ma to 56.0 ± 0.1 Ma)2.
During a period of 4 Ma, several magmatic episodes are recorded in the Akapol pluton, with differing
magma compositions, from mafic to intermediate. Coupled with textural and geochemical evidence for
magma mixing3,5, this magmatism may have occurred in an extensional, back-arc setting with respect to the
UDMA, which may explain its metallogeny. The Akapol pluton had a high volatile content, and it contains
quartz monzonites with miarolitic cavities with small amethyst crystals, Pb skarn mineralization along its
contacts at the Goret Pb-Ag mine, and also high U phases in the eastern part6. Figure 1 Figure 2 A AM1-­‐4 Biotite granite AM1-­‐4 Biotite monzodiorite 57.98 ± 0.31 Ma AM3-­‐4 Bt qz monzodiorite 53.9 ± 2.4 Ma
57.98 ± 0.31 Ma
References
1. Alavi, M., 1994. Tectonophysics, 229, 211–238.
2. Axen, G.J., Lam, P.S., Grove, M. & Stockli, D.F., 2001. Geology, 29(6), 559-562.
3. Esmaeily, D., Khalaj, M. & Valizadeh, M.V., 2007. Iranian Journal of Crystallography and Mineralogy, 1,
169-192. (In Farsi, with English Abstract).
4. Maghdour-Mashhour, R., Esmaeily, D., Tabbakh Shabani, A.A., Chiaradia, M. & Latypov, R., 2015.
Chemie der Erde, 75, 237-260.
5. Sajjadi, M., Vosoughi Abedini, M., Emami, M. & Gorbhani, M., 2011. Quarterly Journal of Earth and
Resources, 3(4), 41-53.
6. Tehrani, M., 1980. Berliner Geowissenschaftliche Abhandlungen, 30, 87 pp.
7. Verdel, C., Wernicke, B.P., Hassanzadeh, J. & Guest, B., 2011. Tectonics, 30, TC3008.
49 PROVENANCE OF THE NEOPROTEROZOIC SIJARIRA GROUP, ZIMBABWE - AN
ANTARCTICA CONNECTION?
S. Mastera,b*, S.M. Glynnb,c & M. Wiedenbeckc,b
a
Economic Geology Research Institute (EGRI); School of Geosciences, University of the Witwatersrand,
Private Bag 3, Wits 2050, Johannesburg, South Africa; *Corresponding Author; Email: sharad.master
@wits.ac.za
b
School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South
Africa
c
Deutsches GeoForschungsZentrum, Potsdam, Telegrafenberg, D-14473, Potsdam, Germany
The Sijarira Group is a Precambrian unfossiliferous, unmetamorphosed redbed sequence, which unconformably overlies the Palaeoproterozoic Magondi Belt in western Zimbabwe. It outcrops in the Chizarira Hills
(where it is up to 660 m thick) and in the Hurungwe area (where it is overthrust by the “Urungwe klippe”,
which contains carbonate rocks and a glacial diamictite, of Neoproterozoic age). Although the
unmetamorphosed Sijarira Group has not been dated, it overlies the Badze granite which has a (reset) Rb-Sr
muscovite age of c. 1211 Ma. It is aligned with, and has been regarded as a continuation of, the TsumisGhanzi belt of early Neoproterozoic redbeds extending across Namibia and northern Botswana. We collected
a sample of coarse-grained trough crossbedded sandstone at 17° 39’ 04.0” S; 27° 51’ 21.0” E, in the
Chizarira Hills. Our sample ZMB13/2 comes from the Ruziruhuru Formation of the Lubu Subgroup,
overlying the basal conglomerates of the Sijarira Group (Humphreys, 1969). We performed U-Pb isotope
dating of detrital zircons from the Sijarira sample using the Cameca 1280-HR SIMS instrument at the
Helmholtz Zentrum Potsdam. The age spectrum that is obtained from the detrital zircons is very surprisingthere are almost no zircons from the underlying Magondi Belt, and none from the Archaean Zimbabwe
Craton, despite an easterly sediment source. Our data indicate that the Sijarira Group is derived from erosion
of a distant late Mesoproterozoic (c. 1.05 Ga) terrain, with minor provenance from Neoproterozoic rocks.
The vast majority (80%) of the zircons record ages between 1075 Ma and 835 Ma. The Sijarira Group cannot
be regarded as a molasse of the nearby Pan-African Zambezi Belt, since it contains very few Neoproterozoic
zircons, and the palaeocurrents were in general from E to W. We suggest that its provenance is from the
erosion of a late Neoproterozoic collisional zone of the southern Mozambique Belt and Maud Belt of West
Dronning Maud Land, Antarctica, to the east of the Zimbabwe Craton. The c. 1075 to 1000 Ma detrital
zircon population in the Sijarira sample could have been sourced from the HU Sverdrupfjella region, in the
Maud Belt, where there are gneisses of that age range formed by late Mesoproterozoic arc collisions (Board
et al., 2005). The youngest Sijarira zircons may be derived from Neoproterozoic intrusive rocks from the
Maud Belt, or they may be related to a 615 ± 30 Ma tectonic event recorded in eastern Zimbabwe. The lack
of Pan-African zircons is due to a relative lack of Pan-African aged magmatism in West Dronning Maud
Land and in the southern Mozambique Belt. Our new maximum age for the Sijarira Group of c. 632 Ma
indicates an Ediacaran depositional age. The Sijarira Group thus correlates in terms of age with other late
Neoproterozoic to Palaeozoic post-Pan-African molasse basins surrounding the Kalahari Craton, such as the
Okwa Group of central Botswana and the Nama basins of southwestern Africa. Its former correlation with
the Ghanzi Group of Botswana cannot be sustained, since the Ghanzi Group has no glacial diamictites, and is
probably pre-Sturtian (i.e., > c. 710 Ma), while the Sijarira Group is constrained to be < c. 632 Ma.
References
Board, W.S., Frimmel, H.E., & Armstrong, R.A., 2005. Pan-African tectonism in the Western Maud Belt: P–
T–t path for high-grade gneisses in the H.U. Sverdrupfjella, East Antarctica. J. Petrology 46(4), 671−699.
Humphreys, M., 1969. The geology of the Western Portion of the Chizarira Game Reserve and adjacent
country, Rhodesia. D. Phil. thesis (unpubl.), Univ. College of Rhodesia, Salisbury, Rhodesia, 297 pp.
50 THE INEVITABILITY OF ANATAXIS DURING PROGRADE METAMORPHISM UNDER
UPPER-AMPHIBOLITE FACIES CONDITIONS
M.J. Mayne 1,2, G. Stevens1, T.E. Johnson3 & J. -F. Moyen2
1
Department of Earth Sciences, University of Stellenbosch, Stellenbosch, South Africa; [email protected]
2
Laboratoire Magmas et Volcans, Université Jean Monnet, Saint-Etienne, France
3
Department of Applied Geology, Curtin University, Perth, Australia
In phase equilibrium modelling it is often impossible to constrain the H2O content of the modelled
composition precisely. Best practice for choosing a realistic H2O content for the rock composition, is
usually to construct an isobaric temperature-XH2O pseudosection and use mineral assemblage and mineral
composition information to constrain the most realistic H2O content for the system. For studies focussed on
the onset of anataxis in rocks with metasedimentary protoliths, the assumption is often made that the
subsolidus rock is in a fully hydrated, fluid absent state and that this results in the first melt being produced
through incongruent melting of micas. Alternatively, it may be assumed that a very small amount of water
exists in the subsolidus assemblage, allowing minor melting as the H2O-saturated solidus is crossed.
Commonly, bulk compositions with water content constrained via a single isobaric T-X section are used to
conduct phase equilibrium modelling over a wide pressure range. This is likely to be problematic as the
H2O contents of both hydrous minerals and melts are pressure dependent. This study uses the ability of the
software package ‘Rcrust’ (Mayne et al., 2016) to conduct phase equilibrium modelling in variable
compositions to investigate the fluid-absent partial melting behaviour of an average pelite and an average
greywacke between 2 and 12 kbar. The results are compared with those produced by assuming that the
water content constrained at a single mid-crustal pressure is relevant to the full pressure range. H2O content
was set by utilising the path dependent iteration tool. This method allows the H2O content of the bulk
composition to be variable, both between and along, each P-T path in an array of paths which define a P-T
plane. At each increment along each path in this plane, the stable assemblage is calculated and if water
exists as a fluid phase it is extracted from the bulk composition. The resulting new, water depleted bulk
composition is used to calculate the stable assemblage at the next increment and the process is repeated.
This method maintains the system at a fully hydrated but fluid-absent state for all P-T paths that progress on
a trajectory consistent with dehydration of the system. Phase equilibrium modelling utilising this new
method on the average pelite and greywacke compositions under investigation indicates that for all but very
steep P-T paths, both rocks melt at the wet solidus, despite the fluid-absent state. The reasons for this can be
understood by examining the water content of the micas, which increases as a function of pressure and
decreases as a function of temperature (Figure 1). Along all but very steep prograde P-T paths the net result
of these two effects is mica dehydration. Consequently, despite the very small temperature increment used
(2 °C) and the precursor fluid- absent assemblage, crossing the wet-solidus results in melt forming as part of
the stable assemblage. It seems that the only way in which a “wet solidus” can be avoided is by following a
P-T path steep enough such that the effect of increasing pressure counteracts the dehydration of the micas as
temperature increases.
a)
b)
Fig. 1: An array of fully hydrated, fluid absent, linear P-T paths originating at 640°C;3kbar steepening with 3°
increments from 0° (with x:y and point resolution set as 2°C:0,0714 kbar). a) The solidus is encountered along P-T
paths at the red open circles; the grey field represents melt present conditions. b) wt.% H2O in Biotite (grey shading)
and wt.% of melt (contour lines) as functions of the length along the P-T path and the P-T gradient (°P/T).
Mayne, M. J., Moyen, J. F., Stevens, G., & Kaislaniemi, L., 2016. Rcrust: a tool for calculating path dependent open system
processes and application to melt loss. Journal of Metamorphic Geology, 34, 663-682.
51 KAROO CFB, SOUTHERN AFRICA-EVOLVED FROM MORB BY MIXING WITH A-TYPE
RHYOLITE IN BIMODAL ASSOCIATION
S. Misra1, A. Smith1 & D. Ray2
1
SAEES, University of KwaZulu-Natal, Durban-4000, South Africa; [email protected]
2
PSDN, Physical Research Laboratory, Ahmedabad- 380 009, India; [email protected]
The ~183 Ma Karoo CFB of southern Africa [1, 2] is a part of the Karoo-Ferrar-SE Australia Large
Igneous Province, which is believed to erupt behind the convergent Pacific margin during the break up of
Pangea between ~200-175 Ma [3]. This CFB, intruding the Archaean Kaapvaal and Zimbabwe cratons,
Palaeo-Proterozoic Limpopo belt and Permian-Jurassic Karoo sedimentary covers, consists of four main
litho-units [4], viz.: (a) minor occurrence of Mashikiri nephelinite volcanics; (b) comparatively large volume
of picritic basalt from northern Lebombo, Nuanetsi and Tuli regions; (c) the largest volume of tholeiitic
basalts and dolerites, which have been subdivided into a Low (L)-Ti basalt (≤ ~2 wt% TiO2) of Lebombo,
Lesotho and southern South Africa and a high (H)-Ti basalt (≥ ~2 wt% TiO2) of northern Lebombo,
Mwenezi and Tuli basins in northern Botswana (along with the Okavango dyke swarm); and (d) the rhyolites
(~183 Ma) capping the basaltic lavas in Lebombo and Mwenezi area. The origin of the Karoo CFB is
uncertain and both mantle plume [5] and plate tectonics [6] models have been proposed. The most-recent
geochemical-cum-isotopic studies suggest a mixed source of sub-continental lithospheric mantle and sublithospheric mantle plume in the genesis of the L-Ti Karoo basalts, and a sub-continental lithospheric mantle
contaminated with a strong Nd-Hf unradiogenic nephelinite-like component (sediment input) for the most of
the H-Ti Karoo basalt [4].
The existing petrogenetic models on the evolution of the CFBs may need revision following the realization
that they are associated with voluminous (> 104 km3) silicic volcanics [7], because the contribution of these
silicic volcanics in the evolution of CFBs has never been explored. A reassessment of literature data and our
new whole-rock analyses (XRF major oxides and LA-ICP-MS trace element data) of the Karoo basalts from
Lesotho, and also basalts as well as rhyolites from Jozini suggest that the Karoo L-Ti basalts are
geochemically similar to MORB particularly in TAS, AFM and Al2O3/TiO2 versus CaO/Al2O3 plots. The
bimodal rhyolites are the volcanic equivalent of A-type granites and likely to be derived from sialic crustal
source(s). The petrographic observations and microprobe analyses on the rhyolites confirm magma mixing
between Karoo basalts and rhyolites, a fact which is also supported by geochemical plots involving major
oxides and trace elements after [ 8, 9]. As the Karoo CFB evolved in an essentially extensional tectonic
setting, the nature of mixing was likely to be anorogenic type. The magma mixing resulted in high variations
in SiO2, K2O/TiO2 and Al2O3/TiO2 ratios in rhyolites, and the formation of H-Ti basaltic magma. Relatively
high proportions of incompatible trace elements, e.g., La, Th, U, Pb, in H-Ti basalts resulted due to downhill
chemical diffusion of these elements during mixing between rhyolitic and L-Ti basaltic parent magmas,
whereas higher TiO2, Sr and Gd (HREE) in the H-Ti basalts could be explained by uphill diffusion of these
elements from a L-Ti to H-Ti basaltic magmas during magma mixing. Our integrated petrographic and
geochemical study suggests that magma mixing had an important role in the geochemical evolution of Karoo
CFB, and imprinted a lithospheric signature on the Karoo basalts. In summary, the Karoo CFB evolved from
MORB-like parent magma through mixing with bimodal rhyolitic magma and does not require a mantle
plume hypothesis.
References:
[1] Jourdan, F. et al. (2005) Geology, 33, 745-748.
[2] Svensen, H. et al. (2012) Earth and Planetary Science Letters, 325, 1-9.
[3] Veevers, J. J. (2012) Earth Science Review, 111, 249-318.
[4] Jourdan, F. et al. (2007) J. Petrology, 48, 1043-1077.
[5] Burke, K. & Dewey, J. E. (1972) J. Geology, 81, 403-433.
[6] Hastie, W. W. et al. (2014) Gondwana Research, 25, 736-755.
[7] Bryan, S. E. (2002) Geol. Soc. America Special Paper, 362, 99-120.
[8] Langmuir, C. H. et al. (1978) Earth and Planetary Science Letters, 37, 380-392.
[9] Cocherie, A. (1986) Geochimica et Cosmochimica Acta, 50, 2517-2522.
52 EARLY-MIDDLE JURASSIC MAFIC DYKES FROM THE H.U. SVEDRUPFJELLA,
ANTARCTICA
Morake, M.A.1*, Knoper, M.W.1, Grantham, G.H.1, Kramers, J.1 and Elburg, M.A1
1
University of Johannesburg, Department of Geology, P.O. Box 524 Auckland Park 2006
*email: [email protected]
The Early to Middle Jurassic break-up of Gondwana produced large-volume magmatic events, resulting in
large igneous provinces (LIPs) such as the Karoo LIP in southern Africa and Ferrar LIP in East Antarctica.
The Early-Middle Jurassic mafic dykes from Sverdrupfjella located in western Dronning Maud Land
(WDML), Antarctica, are regarded as part of the Karoo LIP [1]. These dykes intrude both the Grunehogna
Province (an Archean basement fragment in WDML thought to have been a pre-breakup constituent of the
Kalahari Craton) and the Maud Province (broadly co-eval with the Mesoproterozoic Namaqua-Natal
metamorphic province in southern Africa). The dykes intruding the Grunehogna Province are considered oncraton, whereas those intruding the Maud Province are considered off-craton.
The geochemistry and geochronology of these dykes and basalts found in the Grunehogna Province and the
Maud Province (Vestfjella, Heimefrontfjella and Kirwanveggen) have been studied by previous workers
(e.g., 1) and have been categorized into two groups: low-Ti (TiO2 <2.5%) and high-Ti (TiO2 >2.5%) groups.
Based on 40Ar/39Ar ages of mafic dykes intruding the Grunehogna Province (on-craton), dyke emplacement
occurred at ~178 Ma and ~190 Ma [1]. The Vestfjella basalts (off-craton) have K-Ar ages between 170 and
230 Ma, and plagioclase K/Ar ages at ~180 Ma [2]. The Kirwanveggen basalts (off-craton) yielded a K-Ar
age of 172 ± 10 Ma [3]. Mafic dykes in Sverdrupfjella that intrude Early Jurassic alkaline intrusive bodies
(Straumsvola, Tvora and Jutulröra) show two 40Ar/39Ar age peaks: one at 178-175 Ma (Straumsvola) [4]
and another at 206-204 Ma (Jutulröra) [5]. The dykes from Sverdrupfjella (off-craton) strike dominantly
NNE-SSW, with dip angles ranging from 60° to 90°. The strike trends are similar to equivalent dykes from
the on-craton region of WDML (Grunehogna Province, Almannryggen area) [1].
Samples collected from the Sverdrupfjella are fine to medium grained; the groundmass consists of
plagioclase, augite and minor amounts of magnetite and ilmenite. Phenocrysts consist of plagioclase, olivine
(with inclusions of Cr-spinel) and augite, and pseudomorphs of euhedral olivine and augite. The
geochemistry of the H.U. Sverdrupfjella mafic dykes are characterized by two groups based on Ti content (1)
low-Ti (TiO2 < 2.5%) more dominant, and (2) high-Ti (TiO2 > 2.5%) less dominant. 40Ar/39Ar ages of these
mafic dykes show emplacement ranging from ~211 Ma to ~172 Ma with ~192 Ma and 187 Ma being more
dominant.
[1] Riley T.R. et al. (2005) Journal of Petrology 46:1489-1524.
[2] Luttinen A.V. and Furnes H. (2000) Journal of Petrology 41:1271-1305.
[3] Harris C. et al. (1990) Journal of Petrology 31(2): 341-369.
[4] Riley T.R et al. (2009) Mineralogical Magazine 73:205-226.
[5] Curtis M.L et al. (2008) Journal of Structural Geology 30:1429-1447.
[6] Grantham G.H. (unpubl. data)
53 SURFACE AND SUBSURFACE DESCRIPTIONS OF THE EARLY JURASSIC KAROO
DOLERITES: TOWARD A COMPREHENSIVE UNDERSTANDING OF THE EMPLACEMENT
MECHANISM AND POSSIBLE GAS MIGRATION
Muedi, T1., and Nengovhela, V2
1
Department of Geosciences, AEON, Nelson Mandela Metropolitan University, Port Elizabeth, South
Africa, [email protected]
2
Department of Geosciences, AEON, Nelson Mandela Metropolitan University, Port Elizabeth, South
Africa, [email protected]
This research focuses on the Karoo (182 Ma) dolerites and the possible impact related to their
intrusion on the gas shale and groundwater. Fieldwork was conducted for mapping natural fractures
and outcropping dolerite dyke and sill complexes in the eastern Karoo Basin. This was followed by
re-logging dolerites in the old SOEKOR boreholes drilled in the centre of the basin. The goal is to
analyze these intrusive rocks from different levels (elevations). The number and percentage
thicknesses of the dolerites in each borehole are calculated: OL1/69=23 %; AB1/65 = 24 %;
QU1/65 = 20.40 %; G39856 = 30.50 %; G39980 = 35.10 %; G39974 = 25.5 %; LA1/68 = 33 %;
SP1/69 = 4.5 %. The dolerites thickness ranges from >5 m to 280 m thick from one borehole to
another. In OL1/69 (westernmost studied borehole), thick sills emplaced into the Permian Ecca
Group close to the surface, whilst in borehole SP1/69 (southeasternmost studied borehole) one very
thin dolerite intruded the same sequences at greater depth (4 km). This could be a better site for
shale gas exploration since is intruded with less dolerites. New XRF results from dolerites samples
yielded comparable results from previous analysed dolerites since there are few dolerites close to
the black shale. The cumulative olivine-pyroxenite, possibly ankaramite found at the base of the
Karoo Drakensberg basalt borehole (north easternmost) reveals high concentrations of the nickel
and magnesium. Fig 1. The Karoo geological map showing the study area including locations of the SOEKOR borehole sites.
54 TOWARDS UNDERSTANDING THE EMPLACEMENT MECHANISM OF THE UG-1
CHROMITITE IN THE BUSHVELD COMPLEX
R. Mukherjee1 & R. Latypov1
1
School of Geosciences, University of the Witwatersrand, South Africa; [email protected]
The UG-1 chromitite belonging to the 2.06 Ga old Bushveld Complex (South Africa) has been the subject of
intrigue for petrologists, because of its distinct and spectacular layering with anorthosite (m to mm scale). In
addition this chromitite layer is observed to bifurcate, where the scale of bifurcation varies from mm to 10’s
of cm, and the bifurcations are particularly common for the UG-1 chromitite, compared to the other
chromitite layers in the complex (Nex, 2004; Maier et al., 2013).
The UG-1 chromitite consists of a nearly 1 metre thick massive seam that is underlain by anorthosite in the
footwall, which hosts several thinner UG-1 chromitite sublayers (mm to 10’s of cm). Chromitites from both
these occurrences have been studied to understand about the emplacement mechanism of the UG-1. Field
observations in the Dwars River area (Eastern Bushveld), and in underground platinum mines in the Western
Bushveld, revealed several interesting features of the UG-1 chromitite like: intrusion of chromitite into
anorthosite that lead to ripping of anorthosite fragments, which though detached, remain close to the main
anorthosite layer, truncation of thinner (mm-scale) chromitite layers within anorthosite against relatively
thicker (40 cm) chromitite layers, potholes, and truncation of layering in anorthosite defined by pyroxene
oikocrysts by UG-1 chromitite layers. All these features collectively indicate that one of the mechanisms of
the UG-1 chromitite was its emplacement as sills within the anorthosite.
The bulk rock geochemical data (major and trace elements, and platinum group of elements: PGE) also adds
insight about the emplacement mechanism of the UG-1 chromitite. The major and trace elements, and PGE
show cyclical patterns in their distribution, in an otherwise uniform massive chromitite seam which is nearly
1 metre thick. The inferences were derived by studying drill cores from the western as well as the eastern
limbs of the Bushveld Complex. Similar distribution of geochemical trends displayed by the trace elements
and PGEs in chromitites, which are separated by several 100’s of kilometres, indicate that at least three
magmatic replenishment events were responsible for forming the massive UG-1 chromitite seam. Melts of
almost similar composition was entering the magma chamber for forming discrete chromitite layers that
accumulated to form the massive UG-1 chromitite seam. The chromite chemistry (Mg#: 21‒38; Cr#: 64‒74)
indicates a parental melt which was slightly evolved but enriched in Cr2O3, akin to boninites. Chromite
composition is observed to progressively increase in MgO and Al2O3, and decrease in Cr2O3 and TiO2
towards the bottom of the massive seam. This probably indicates the more orthocumulate nature of the
chromitite layer towards the bottom of the seam, where interstitial silicates like orthopyroxene and
plagioclase dominate.
In conclusion, the field observations and the geochemical data may be integrated to suggest multiple modes
of emplacement of the UG-1 chromitite. Some of the UG-1 chromitite layers occurring within anorthosite
may have formed as sills, while the massive seam was composed of several discreet chromitite layers that
formed through at least three cycles of magma influx.
Nex, P.A.M. (2004). Formation of bifurcating chromitite layers of the UG1 in the Bushveld Igneous
Complex, an analogy with sand volcanoes. Journal of Geological Society of London 161:903–909.
Maier, W.D., Barnes, S-J, Groves, D.I. (2013). The Bushveld Complex, South Africa: Formation of
platinum palladium, chrome- and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate
slurry in a large, relatively slowly cooling, subsiding magma chamber: Mineralium Deposita 48: 1–56.
55 PETROGENESIS OF THE LATE ARCHEAN SINGERTÂT ALKALINE IGNEOUS COMPLEX,
NORTH ATLANTIC CRATON, SOUTH-EAST GREENLAND
A. Naidoo1, G.M. Bybee1 & S. Tappe2
1
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;
[email protected]; [email protected]
2
Department of Geology, University of Johannesburg, Auckland Park, South Africa; [email protected]
The Singertât Complex forms part of the Skjoldungen Alkaline Igneous Province (SAIP) located in SE
Greenland. It was emplaced into the southern part of the SAIP in a sequence of small sheet-like intrusions at
ca. 2664 Ma. The complex consists of a variety of silica undersaturated alkaline rocks (e.g., ijolites and
micaceous pyroxenites), which occur as modally layered horizontal sheets cut by minor late-stage nepheline
syenitic pegmatite and carbonatite sheets and stocks. An overarching aim of this project is to understand the
origins and evolution of the magmas that formed the Singertât Complex. An understanding of the
petrogenesis of the complex will include understanding the processes that the magma(s) may have
experienced from source to final emplacement level, magma and crystal dynamics, as well as the evolution
of the crystal cargo in the system. This will be achieved firstly by determining whether the complex is
constructed from multiple magma pulses or a single pulse. Secondly, whether those magma pulses were
emplaced as crystal-poor liquids or as crystal mushes. Thirdly, determining the processes of crystallisation
(gravity settling or in-situ crystallisation) under which the magma chamber was operating. To achieve the
project goals, analyses such as extensive mineral and textural identification, X-Ray tomography, electron
microprobe analysis and laser ablation ICPMS will be conducted. Detailed petrographic analysis indicates
that the Singertat Complex is composed of nepheline-bearing lithologies such as urtite (>70% nepheline),
ijolite (between 70%-30% nepheline) and melteigeite (<30% nepheline). The main mineral phases observed
are nepheline, alkali feldspar, aegerine-augite, biotite and minor primary carbonate. Poikilitic relationships
between clinopyroxene, biotite and nepheline are observed. Although this may not be the case in all the
nepheline-bearing rocks from Singertât, the rock types in which this poikilitic texture occurs would suggest
that the mafic minerals crystallized first as euhedral crystals followed by nepheline which appears to have
crystallized from the remaining interstitial liquid. Nepheline commonly exhibits kalsilite exsolutions in a
perthite-like texture, which indicates supersolvus crystallization and very slow cooling of the intrusive body.
Certain samples contain polygonal grain boundaries between minerals such as nepheline and clinopyroxene,
providing evidence for pressure solution and overgrowth during compaction. In many of the samples a
gradational transition from one rock type to another is observed, however in other samples there appears to
be a sharp transition between rock types (observed in hand sample). Our petrographic investigation shows
that these boundaries are truncated suggesting that the underlying unit has been chemically or thermally
eroded. An alternative explanation could be that these sharp contacts are a result of late-stage sheets or sills
cutting pre-existing rock layers. Pronounced grain size layering also occurs in many of the samples. These
changes in grain size are often on a millimeter and occasionally a centimeter scale. Flow textures, such as the
alignment of clinopyroxene, biotite, nepheline and alkali feldspars are observed. Our preliminary
petrographic evidence indicates that the small peralkaline complex was most likely formed due to multiple
magma pulses of crystal-rich magma with subsequent in-situ crystallization as the primary processes of layer
formation in the magma chamber.
56 A petrographic, geochemical and geochronological study of mafic sills within the Transvaal
Supergroup, east of the Bushveld Igneous Complex.
Thendo Netshidzivhe, Klausen, M.B.
Department of Earth Sciences, Univ. of Stellenbosch, Matieland 7602, South Africa, [email protected]
Keywords: Bushveld Complex, Baddeleyite, geochronology, Uitkomst Complex
The ~9 km thick Rustenburg Layered Series (RLS) of the enormous Bushveld Igneous Complex (BIC) is the
World’s largest known ultramafic-mafic layered intrusion, where it has shown that its Marginal Zone (MZ,
~2056 Ma), Lower Zone (LZ) and Critical Zone (CZ; ~2055 Ma) crystallized within ~1 Ma [1]. For this
‘Large Igneous Province’, it has been inferred that so-called B1, B2 and B3 type marginal sills, represent
feeders/parents to the RLS’ (1) LZ to Lower CZ, (2) upper CZ, and (3) Main Zone (MZ), respectively [2-3].
While most previous studies have focused on BIC’s more proximally located marginal sills, this paper
reports new geochemical, petrographical as well as U-Pb geochronological data on more distal sills that are
also hosted stratigraphically deeper into – and even below – the Transvaal Supergroup.
We find that typical ‘boninitic’ B1-sills (with higher Mg-Si and lower Fe-Ti; cf., Figure) were emplaced
throughout this sedimentary sequence, together with associated more differentiated varieties (with
interstitial-graphic quartz) as well as orthopyroxenites (only in the Magaliesberg Fm). Arguably, slightly
older (2058±6 Ma; [4]) B1 sills are consistent with initial parents
being injected as a more extensive and pervasive sill complex
prior to the initial growth of the BIC magma chamber. This
model interpretation will be tested by additional baddeleyite UPb
ages of sampled B1 sills.
In contrast, no B2 and B3 sills – as defined by [3] – are found
along our sample traverse, but rather more tholeiitic sills (with
lower Mg-Si and higher Fe-Ti; cf., Figure) that cluster within
Silverton’s Bowen Member as well as below the basal
Protobasins (including Uitkomst). Thus, we are inclined to
believe that all of our more tholeiitic dolerites represent younger
intrusions. This is supported by the presence of geochemically
similar ~1.85 Ga Black Hills dykes [5] – which cut across our
study area and are hosted as associated sills within the nearby
Waterberg Group [6] – but could, of course, also include sills that
were emplaced during an even younger Umkondo [7] or Karoo
event. U-Pb ages on baddeleyite extracts from two
Strubenkophosted tholeiitic sills and a gabbro from the Uitkomst
Complex will be able to test whether tholeiitic sills were syn-BIC
feeders or belong to any of the above-mentioned younger
magmatic events.
Figure: Bulk rock compositions of sampled sills compared to
B13 magmas [3]. Solid lines separate a ‘boninitic norite’ (BN) suite from ‘tholeiitic dolerites’ (TD). Dashed
lines delineate typical boninitic compositions. Arrows point towards three cumulate samples belonging to
each suite.
References:
[1] Zeh A, Ovtcharova, M, Wilson, AH & Schaltegger, U (2015) Earth Planetary Science Letters 418: 103-114
[2] Sharpe, MR (1981) Journal of the Geological Society, London 138: 307-326
[3] Barnes S, Maier, WD & Curl, EA (2010) Economic Geology 105: 1491-1511
[4] Wabo, H, de Kock, MO, Klausen, MB, Söderlund, U, Beukes, NJ (2015): GFF, 138: 133-151
[5] Olsson, JR, Klausen, MB, Hamilton, MA, März, N, Söderlund, U, & Roberts, RJ (2015) GFF 138: 183-202
[6] Hanson, RE, et al. (2004) South African Journal of Geology 107: 233-254 [7] de Kock, MO et al. (2014)
Precambrian Research 249: 129-143
57 CONSTRAINTS ON THE EMPLACEMENT OF THE COLESBERG SILL, KAROO
SUPERGROUP
M. Ntantiso and S.A. Prevec
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected]
The existence of so-called saucer-shaped sills is a distinctive feature of the Karoo Supergroup, and the
emplacement mechanisms of these sills have been the source of regular inquiry since they were first
recognised by du Toit. A wide range of emplacement models have been proposed, accordingly. In a study of
the outer ring of a large sill emplaced in the Colesberg area, Northern Cape Province, the petrological and
geochemical variations have been examined.
The sill, which is ca. 150 m thick and intrudes flat-lying Karoo sandstones and shales of the Beaufort Group,
is characterised by a quenched upper contact containing euhedral phenocrysts of pyroxene and plagioclase
within a nearly aphanitic groundmass. The lower contact is not exposed, although it is believed to be close to
the end of the exposed outcrop. The interior of the sill features olivine, locally, as well as orthopyroxene, in
addition to the minerals mentioned, in a medium- to coarse-grained plutonic textured rock. The geochemical
stratigraphy of the sill shows a pattern broadly consistent with a so-called D-shaped profile, with top and
bottom marginal ‘reversals’. The “D” is defined by increases in the concentrations of Mg and Fe, but
corresponding decreases in Si, Al, and Ca. The incompatible major, minor and trace elements (e.g., Ti, P, K,
Na, and the LILE and HFSE) follow the pattern defined by silica, broadly decreasing through the interior of
the sill and more abundant near the margins. The strongly siderophile elements, conversely, show maxima in
the sill interior, with the exception of V and Sc, which are interpreted as following their oxide affiliations.
Cu, Zn and Pb behave in the manner of the incompatible elements. These trends are broadly consistent with
an increase in the proportions of Mg-Fe minerals, such as olivine and orthopyroxene, towards the centre of
the sill. The apparent absence of these phases in the chilled top margin rocks suggests that this reflects a
changing magma composition over time, rather than physical mechanical enrichment of early-crystallising
minerals by flow differentiation, for example.
The upper and lower contacts show behaviour independent of the overall “D” profile, in general, on the scale
of about 30 m in both cases. The elemental concentration variations are, however, not necessarily
symmetrical when comparing upper and lower contacts. For example, while Mg, Ti, Na, Cu, Ni, Pb, and Sc
show symmetrical increases or decreases across the top and bottom margins, many elements (including Si,
Fe and the HFSE and LILE) show sharp decreases below the top contact, whilst showing increases upwards
from the base. Superimposed on all of this is an anomalous but significant enrichment in Al and the
incompatible elements in one sample at 54 m depth (about one third of the distance from the top).
Both ortho- and clinopyroxene show trends of least evolved compositions near the base of the sill, and more
evolved near the top. Plagioclase shows no such general trend, where the more primitive plagioclase
compositions (around An85) occur near the sill interior, and in the top chilled margin. However, while in the
top half of the sill the plagioclase rims are more evolved than the cores, consistent with crystallisation of
trapped interstitial liquids, in the lower half of the sill the rims are typically more primitive than the cores.
A model is tentatively proposed involving emplacement and rapid cooling at the contacts, followed by
crystallisation of early phenocrysts, and progressive evolution of in situ trapped liquids to more evolved
compositions (preserved at the base, facilitated by upwards migration of interstitial liquids as crystallisation
proceeded). The interior of the sill was dominated, or overprinted, by successive emplacement of primitive
(parental) magma, which infiltrated the existing crystal mush particularly in the liquid-dominated sill
interior. Late, primitive plagioclase rims would require downward settling of this later, dense liquid into the
existing mush below. Near the top margin, incompatible elements appear to have been preferentially
extracted from the top 30 m to be reconcentrated in a narrow zone just below, best explained by migration of
interstitial melt.
58 PETROGENESIS OF THE MOUNT AYLIFF COMPLEX: EVIDENCE FROM CR–SPINEL
GEOCHEMISTRY.
B.I. Ntsaluba and S. A. Prevec
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected]
The Mount Ayliff Complex (MAC) is a composite, transgressive mafic sill located between the border of
Kwa-Zulu and the Eastern Cape, approximately 10 km south of Kokstad, and is believed to represent feeders
to the Karoo Large Igneous Province (LIP). The MAC outcrops as five distinct lobes that are believed to
have formed a continuous intrusive sheet prior to erosional dissection. These five lobes include: Insizwa,
Ingeli, Tabankulu, Tonti and Horseshoe lobes.
Cr-spinels are common accessory minerals in the Basal Zone of the Mount Ayliff Complex. In this study, Crspinels from the olivine-rich rocks from the Insizwa, Tonti and Tabankulu lobes were analysed via electron
microprobe. These chromian spinels show distinctive compositional characteristics. These variations include:
Cr2O3 (27.47 – 47.48 wt%), MgO (2.18 – 11.02 wt%), Al2O3 (6.86 – 19.29 wt%) and TiO2 (0.86 – 8.62%).
Cr-spinels in the Insizwa plagioclase-bearing lherzolite (PBLH) unit show consistently low Cr+Al cation
proportions for the same range in Ti as compared to the PBLH unit in the Tonti and Tabankulu lobes.
Conversely, Insizwa PBLH Cr-spinels show elevated Fe3+/(Fe3++Cr+Al) ratios as compared to the Tonti and
Tabankulu lobes. Cr-spinels in the Insizwa PBLH unit overlap with those from the troctolite (TRCT) unit
while the Tabankulu PBLH and TRCT units show distinct clusters.
The anomalously high Fe3+ values in the Insizwa PBLH unit as compared to the Tonti and Tabankulu lobes
are suggestive of an extensive reaction with trapped liquid in the former. The overlap of the Cr-spinels from
the PBLH and UOGN units in the Insizwa lobe as opposed to the distinct clusters observed in the Tabankulu
lobe suggest that the Tabankulu TRCT unit was sourced and fed from the Insizwa lobe. The observations
have implications for intra-lobe processes and emplacement models suggested for the Mount Ayliff
Complex.
Multi element variation diagrams for olivine-rich rocks in the Basal Zone of the Insizwa, Tonti and Tabankulu
lobes. Left: Plot of TiO2 (wt%) vs Fe3+/(Fe3++Cr+Al). Right: Cr+Al vs. Ti (cation proportion). Cation
proportions have been recalculated to 32 oxygen atoms.
59 O-ISOTOPE IMPLICATIONS FOR THE MAGMA ORIGIN OF CO-EXISTING GRANITES AND
NEPHELINE SYENITES OF THE DITRĂU MASSIF, ROMANIA
A. Odri1 & C. Harris1
1
Department of Geological Sciences, University of Cape Town, Cape Town, South Africa;
[email protected], [email protected]
The Ditrău Alkaline Massif (DAM) is a Mesozoic alkaline igneous complex situated in the Eastern part of
Carpathian Mountains (Romania). The DAM is an intrusive complex, which was initially generated in an
extensional, rift-related environment in an intraplate setting during the Late-Triassic Tethyan extension
(Săndulescu, 1984). A wide variety of igneous rocks have been described in the massif (ultramafic rocks,
gabbros, diorites, monzonites, syenites, nepheline syenites, and granites) however, agreement on the
petrological evolution of the DAM after 200 years of investigation is still rather poorly understood, mainly
due to the absence of comprehensive isotope data. As a consequence of this, the relationship between silica
under- and over-saturated felsic rocks has not yet been fully explained. The aim of this study to reinvestigate their origin and relationships mainly on the basis of O-isotope ratios of minerals, combined with
whole-rock radiogenic isotope data. The current hypotheses concerning the origin of the DAM suggests
fractional crystallization of a mantle-derived magma perhaps accompanied by minor crustal contamination in
the case of Si-oversaturated rocks (Morogan et al., 2000, Pal-Molnar et al. 2000), although the assimilation
has not been properly investigated so far. The main constituents of granites are quartz, K-feldspar,
plagioclase, subordinate biotite ± ferro-edenite and -hornblende. Syenites are composed of K-feldspar,
plagioclase, biotite, ferro-pargasite and hastingsite. Nepheline syenites contain K-feldspar, plagioclase,
biotite, nepheline, aegirine and aegirine-augite as main constituents. The O-isotope measurements were made
on quartz, zircon and feldspar separates from the granites (δ18O(qtz)=10.2-12.9‰ (n=8); δ18O(zrc)= 5.98-7.22‰
(n=5); δ18O(fsp)=8.42-10.77‰ (n=8)), syenites (δ18O(zrc)=6.01-6.26‰ (n=2)) and nepheline syenites
(δ18O(zrc)=5.11-5.26‰ (n=2)). On the basis of the δ18O values it can be concluded that whereas the nepheline
syenites fit into the current hypothesis with their mantle-like δ18O values, crustal involvement was significant
in the formation of syenites and granites. The ɛSr and ɛNd values of the granites range from 18.3 to224
(n=3), and 1.1-3.4 (n=5), respectively. Syenites have mainly negative ɛSr (-26.6-0.23; n=2) with more
positive ɛNd values (4.1-5.2; n=2), whereas the ɛSr values of nepheline syenites are from 30.5 to 70.9 (n=2)
and the ɛNd from 5.1-5.3 (n=2).
Radiogenic isotope data are, therefore, consistent with mantle-derivation for all the felsic rocks of
the DAM, possibly with a minor contamination. This is in contrast to the O-isotope data for granites and
syenites which are consistent with a high crustal input. The O-isotope data indicate that there is no direct
petrogenetic relationship between the nepheline syenites and the other felsic rocks. Based on the isotopic
data, two models are proposed for the generation of granites and syenites: (1) partial melting of crust
metasomatised by mantle fluids or (2) crustal contamination of mantle-derived magma by a contaminant
with high δ18O and low Sr and Nd.
References:
Săndulescu, M. (1984): Geotectonica Romaniei. [Geotectonics of Romania.] Editura Tehnică, Bucureşti
Pal-Molnar, E. (2000): Hornblendites and diorites of the Ditrău Syenite Massif. Department of Mineralogy,
Geochemistry and Petrology, University of Szeged.
Morogan, V., Upton, B.G.J., Fitton J.G. (2000): The petrology of the Ditrău alkaline complex, Eastern
Carpathians. – Mineralogy and Petrology, 69, 227–265.
60 THE STRUCTURAL GEOLOGY, METAMORPHISM AND GEOCHRONOLOGY OF THE
ZWARTKOPS HILLS
R.J. Ormond1, J. Lehmann2 & G.A. Belyanin3
Department of Geology, University of Johannesburg, Auckland Park, South Africa;
1
[email protected]; 2 [email protected]; 3 [email protected]
The Zwartkops Hills located along the north-western margin of the Johannesburg Dome, have been
identified in the past as a deformed outlier of the Witwatersrand Supergroup resting on Archean rocks. The
mechanisms leading to formation of this outlier remain poorly studied. The structurally lower sequence
consists of metadiorite, felsic pegmatite, and leucogranite as well as chlorite schist. The overlying cover is
composed of a supracrustal package of metamorphosed siliciclastics including orthoquartzite, metasiltstone
and schist.
The basement metadiorite records early deformation D1 indicated by steep N-striking gneissic foliation S1
and steep aggregate mineral lineation L1 and is associated with amphibolite facies metamorphism M1. The
cover sequence has been affected heterogeneously by biotite-grade, greenschist facies metamorphism M2 that
is mostly developed in the schists during D2. The greenschist facies metamorphism M2 is also associated
with the formation of chlorite schist of the basement. Asymmetric, open to tight, inclined folds F2 of north
and south vergence, shallow-plunging crenulation lineation L2 parallel to the F2 fold axes occurs within the
rocks of the cover and the chlorite schist of the basement. S2 schistosity that is nearly parallel to the longlimb of the south verging F2 folds, as well as down-dip mineral lineation on the S2 schistosity is developed in
the chlorite schist of the basement. The S2 schistosity is also characteristic of the schist of the cover.
S2 schistosity and F2 folds are developed in the chlorite schist of the basement, but no further evidence of D2
deformation is noted in the other lithologies of the basement. Refolding of the schistosity S2 occurs in the
cover schists due to NNW – SSE compression, in the form of conjugate contractional kink folds and chevron
folds, is interpreted as progressive deformation D2 during exhumation.
40
Ar/39Ar stepwise heating plateau ages were obtained from white mica, separated by settling and/or handpicking. A sample containing 0.4-3mm sized white micas collected from a shear zone recording green-schist
facies metamorphism developed within the metadiorite. It gave a 40Ar/39Ar plateau age at ca. 2140 Ma,
interpreted as the age of shearing. 40Ar/39Ar plateau ages of ca. 2016-2026 Ma were obtained from the
leucogranite (white micas sized ± 2 mm), a sample from the tectonized contact between the leucogranite and
chlorite schist of the basement (containing <100 µm sized white micas) and from a schistosed cataclasite
(<100 µm sized white micas). This suggests that the formation of the Vredefort Impact Structure (VIS),
nowadays located ca. 100km to the southwest of the Zwartkops Hills, played a role in the deformation
history of Zwartkops.
The findings suggest two unrelated deformation events, D1 and D2. Deformation D2 of the cover at depth
during N-S crustal shortening is thought to have occurred along a sole décollement represented by the weak
chlorite schist. The D2 event can be further separated into a higher-grade greenschist facies metamorphism
period of deformation and lower–grade deformation during exhumation, forming the S2 schistosity and the
subsequent refolding of the S2 fabric respectively. The relation of the shearing at ca. 2140 Ma in the
basement, to that of the cover and the D2 deformation event as well as the relationship between the D2
deformation event and the VIS remains poorly constrained. 61 MAGMA MUSH DYNAMICS IN THE KUNENE ANORTHOSITE COMPLEX, ANGOLA
T. Owen-Smith1, B. Hayes2, G. Bybee2, J. Lehmann1, L. Ashwal2, K. Hill2 & A. Brower2
1
Department of Geology, University of Johannesburg, Johannesburg, South Africa; [email protected]
2
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Proterozoic massif-type anorthosites are voluminous, plagioclase-dominated composite intrusions that occur
around the world, but during a restricted period in time, mainly between 1 and 1.8 Ga. Despite several
decades of research, there remain many fundamental questions regarding the formation of this enigmatic
rock type. One of these outstanding issues concerns the mechanism of emplacement, i.e., what form do these
magmas take and how do they rise through the crust? It is largely agreed that these are crystal-rich magmas
with a polybaric fractionation history, but there is still no consensus on the degree of crystallinity, the mode
of ascent and emplacement, or the timescales involved.
The 1.44–1.38 Ga Kunene Anorthosite Complex (KAC) of northern Namibia and southern Angola is one of
the largest anorthosite complexes in the world (~18 000 km2), yet relatively poorly studied. Unlike many
complexes of this type, the KAC rocks preserve fresh igneous textures, allowing for direct investigation of
the magmatic history of the system. We present field and petrographic evidence from the KAC for
incremental growth by emplacement of (and interaction between) multiple, compositionally diverse pulses of
crystal-rich magma; localised disaggregation and remobilisation of pre-existing crystal mushes and
anorthositic material; and late-stage melt mobility and channelisation. Non-cotectic plagioclase proportions
and evidence for extensive mechanical abrasion and recrystallization of cumulus plagioclase crystals
suggests mush emplacement at crystallinities greater than 50%, and possibly as high as 95%., i.e., beyond the
so-called “lock-up point” at which magma is assumed to behave as a solid. Our findings have important
implications for the understanding of the behaviour of crystal mushes in igneous systems, and on the
mechanisms of crustal differentiation in Proterozoic times. 62 FENITES FROM THE UPPER ZONE & NEBO GRANITE OF THE BUSHVELD COMPLEX IN
THE AUREOLE OF THE SPITSKOP COMPLEX
A. Ozturk1, P. Nex2 & J. Kinnaird2
1
School of Geosciences, University
[email protected]
of
the
Witwatersrand,
Johannesburg,
South
Africa;
The Spitskop Complex is an intrusive carbonatite-alkaline rock complex which was emplaced into the
Bushveld Igneous Complex on the Kaapvaal Craton at approximately 1.3 Ga. It consists of primarily
carbonatites, nepheline syenites and ijolites with pyroxenites, surrounded by the country rocks of the Upper
Zone of the Rustenburg Layered Suite and granites of the Lebowa Granite Suite. The carbonatite is divided
into three different compositional zones: calcite carbonatite, calcite-dolomite carbonatite and dolomite
carbonatite with an apatite rich zone. There is no economic REE mineralization at Spitskop.
In the aureole of the Spitskop Complex fluids from the emplacement of carbonatites and/or alkaline silicates
have widely metasomatised the Lebowa Granite Suite country rocks and rocks from the Upper Zone of the
underlying Rustenburg Layered Suite. SEM results suggest that these fluids reacted with nepheline syenites,
and formed analcime after nepheline. Additionally SEM images show a variety of reaction rims and element
replacements, which are one of the most important evidences of the fenitisation process. The mineralogical
composition of fenites have been determined and the vein relationships are analyzed. This SEM investigation
shows that some of the fenitised Nebo granites resemble syenites and that the Upper Zone ferrogabbros
resemble ijolites.
Figure 1: Geological map of the Spitskop Complex showing the distribution of carbonatite, nepheline
syenite, ijolite and fenitized rocks surrounded by Bushveld granite (Gose et al., 2013).
References:
Gose, W. A., Hanson, R. E., Harmer, R. E., Seidel, E. K. (2013). Reconnaissance paleomagnetic studies of
Mesoproterozoic alkaline igneous complexes in the Kaapvaal Craton, South Africa. Journal of African Earth
Sciences, 85, 22-30.
Harmer, R. E., (1999). The petrogenetic association of carbonatite and alkaline magmatism: Constraints from
the Spitskop Complex, South Africa. Journal of Petrology 40, 525–548.
63 UNRAVELLING THE CONUNDRUM OF BIFURCATING CHROMITITE LAYERS IN THE
BUSHVELD COMPLEX
M.A. Pebane1,2 and R. Latypov2
1
Mineralogy Division, Mintek, Randburg, South Africa; [email protected]
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
[email protected]
2
The UG1 chromitite refers to a package the stratiform chromitite layers hosted in the Upper Critical Zone of
the Bushveld Complex. The bifurcation of UG1 chromitites within anorthosite, i.e. successive convergence
and divergence of chromitite layers along strike, remains an unresolved petrological enigma of the Bushveld
Complex. The bifurcation has been variously interpreted as resulting from: a local accumulation of
anorthosite lenses, repeatedly terminated by crystallization of continuous chromitite layers (a depositional
model); late-stage injections of chromite-rich mush along anastomosing fractures within the anorthositic pile
(an intrusive model); post-depositional slumping of an anorthosite pile, resulting in local merging of
chromitite layers (a deformational model). Testing of these models against field relations at Dwars River
locality resulted in the following most telling observations: highly irregular and scalloped contacts between
anorthosite and chromitite layers; abrupt lateral termination of thin anorthosite layers within chromitites; in
situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitites;
transported anorthosite blocks within chromitites; disrupted anorthosite-chromitite layers overlain by planar
non-deformed chromitite layers; sill-like protrusions of chromitites into footwall anorthosites. None of the
suggested models for the origin of the bifurcating chromitite layers are consistent with all of these relations.
We propose a novel hypothesis that envisages replenishment of the chamber by basal flows of new dense and
superheated magma that caused intensive thermo-chemical erosion of the footwall anorthosite-chromitite
sequence. The melting and dissolution of anorthosite was patchy and commonly terminated upon reaching
refractory chromitite layers, resulting in erosional lens-like remnants of anorthosite resting on continuous
chromitite layers. With cooling, the magma crystallized new chromitite layers directly on the irregular
chamber floor, draping over all erosional remnants of anorthosite and merging with the pre-existing
chromitite layers locally excavated by erosion. With further cooling the magma crystallized overlying
anorthosite (chromite-plagioclase cumulate). Emplacement of multiple pulses of magma led to a repetition of
this sequence of events, resulting in a complex package of anorthosite cumulates hosting numerous
bifurcating chromitite layers. This erosional hypothesis for the origin of bifurcating chromitite layers appears
to provide the simplest explanation for most of the field features of this enigmatic igneous layering of the
Bushveld Complex.
64 CONTAMINATION OF MAFIC MAGMAS: THE ROLE OF HYDRATION, OXIDATION, AND
SILICEOUS CRUST
S.A. Prevec
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected]
There is strong evidence based on geochemical, isotopic, and petrographic grounds for the correspondence of
crustal contamination with mineralisation, both for magmatic sulphide ores and for magmatic oxide ores,
including both chromite and magnetite. Assimilation of carbonate footwall is a popular model for the
generation of Fe-Ti-V ores, while evidence of radiogenic siliceous and alkaline crust is commonly associated
with chromitites and associated disseminated Cu-Ni-PGE-sulphide ores. However robust the association, it
remains entirely unclear as to the cause and effect relationship between many of these phenomena.
The most commonly proposed mechanism for inducing magma oxidation, and the resultant precipitation of
Fe-Ti-V oxides, or (less commonly-proposed) Fe-Cr oxides, is through assimilation of footwall carbonates,
commonly associated with many large igneous provinces. At conditions at or near the QFM buffer, there is a
strong correlation between more oxidised melts and earlier oxide mineral crystallisation. However, it is not
entirely clear that carbonate breakdown will generate O2 (as opposed to CO2, which might be expected to
degas from/through, and not react with, the magma), nor that this will actually facilitate a shift in the redox
state of the magma. Carbonate assimilation is energetically and frequently logistically practical, but it is not
easily identified by common monitors of crystal assimilation such as radiogenic isotopic or HFSE
geochemical shifts. Furthermore, unless the carbonates are also sulphur-rich, magma oxidation will increase
the magma’s capacity to dissolve S, decreasing the likelihood of observed oxide-sulphide associations. The
relative availability of carbonate footwalls to layered intrusions, and the documented presence of carbonaterich phases associated with some reef-type mineralisation, supports this association.
The most commonly proposed mechanisms for inducing chromite precipitation in mafic magmas involves
either magma mixing or contamination by siliceous crust. It has been demonstrated, however, that the alkali
content of granitoid crust renders this unhelpful in inducing chromite-only precipitation (e.g., Irvine, 1973),
as the alkalis stabilise the olivine at the expense of chromite. Similarly, it has been demonstrated that magma
mixing (between primitive and evolved versions of the same magma) will not induce sulphur saturation (e.g.,
Cawthorn, 2002). The presence of distinct radiogenic isotopic shifts and of alkaline inclusions in earlyformed minerals supports this association.
The enhancement of magmatic water contents, though either assimilation of wet materials, or crystallisation
of dry minerals, or a combination of both, will also enhance the stability of oxide minerals, particularly of
magnetite at the expense of ilmenite, but spinels in general relative to most coexisting silicates. Higher water
contents also facilitate the dissolution and transport of sulphur-compounds. The presence of chrome spinel
layers associated with varitextured (taxitic) rocks, olivine- and clinopyroxene-enriched pegmatoids, supports
this association.
Quantitative modelling of magma hydration and oxidation via AFC processes involving silicate and
carbonate contaminants is proposed as a preferred method of facilitating enrichment in spinel abundances in
magmas prior to emplacement, as an alternative to in situ spinel generation and accumulation.
References:
Cawthorn, R.G. (2002) Economic Geology 97: 663-666.
Irvine, T.N. (1973) Carnegie Institute of Washington Yearbook 73: 300-316.
65 ARCHEAN SUBDUCTION RELICS IN THE CRATONIC ROOT–EVIDENCE FROM ECLOGITE
XENOLITHS
Radu, I.B.1,2, Harris, C.2, Moine, B.N.1, Boyet, M.3, Costin, G.4 & Cottin, J.-Y.1 1
Univ. Lyon, UJM Saint-Etienne, UBP, CNRS, IRD, Laboratoire Magmas et Volcans UR 6524, F-42023 Saint-Etienne, France, [email protected] 2
Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa,
[email protected] 3
Laboratoire Magmas et Volcans, Univ. Blaise Pascal-CNRS-IRD-OPGC, 63038 Clermont-Ferrand, France
4
Department of Earth Science, Rice University, Houston, TX 77005, USA, [email protected] The origin of mantle eclogites is under constant debate, and most often interpreted as derived from a
subducted oceanic crust. The present study focuses on the petrological and geochemical characteristics of a
widely sampled suite of eclogite xenoliths from Roberts Victor Mine, South Africa. Non-metasomatised
samples are preserved in the lowermost part of the cratonic root, corresponding to depths of 145-200km.
These samples, variable in textures, are bi-mineral, kyanite- or corundum-bearing. The high-Al eclogites
show low REE abundances and pronounced positive Eu and Sr anomalies. The δ18O values of cpx and garnet
show a high degree of correlation (Fig. 1) and a wide range of values (1.36 to 6.60 ‰). Most bi-mineral
samples equilibrated around 950°–1000°C at 5GPa, whereas the corundum-bearing eclogites equilibrated at
higher temperatures (1400°–1450°C) and pressures (6.5GPa) according to thermo-barometric calculations [1]
with the 39mW local geotherm [2]. Reconstructed whole rocks have the REE distribution of a depleted
gabbro (for high-Al eclogites) and of an associated websterite (for bi-mineral eclogites), which together with
the δ18O values prove an oceanic crustal origin for mantle eclogite xenoliths. Two of the bi-mineral eclogites have
Meso- to Neo-Proterozoic ages, while
the two corundum-bearing samples have
Paleo- to Meso-Proterozoic ages defined
by the two 176Lu–176Hf and 147Sm–143Nd
isotope systematics respectively. This is consistent with the subduction of
a hydrothermally altered, gabbro to
high-Al websteritic sequence of a
depleted oceanic crust. We infer that
plate tectonic processes similar to
phanerozoic subduction were already
shaping the Earth at the end of the
Archaean. However, the δ18O values of
some samples are lower than those
found in present day oceanic crust and
in Cretaceous ophiolites. These low δ18O
values may reflect higher temperatures of
seawater ocean crust exchange.
Figure 1. Plot of δ18O of garnet - grt (red), kyanite - ky
(light blue) and corundum – cor (dark blue) vs δ18O of
clinopyroxene - cpx in non-metasomatized and
metasomatized (grey) samples [1]
Krogh-Ravna (2000), Journal of
metamorphic Geology 18, 211–219 [2]
Griffin et al. (2003), Lithos 71, 215–241
66 RESOLVING TEMPERATURE-TIME RELATIONS IN METAMORPHIC ROCKS USING INSITU GEOCHRONOLOGY
J. Reinhardt1 & A. Zeh2
1
Department of Earth Sciences, University of the Western Cape, Bellville 7535, South Africa;
[email protected]
2
Institut für Geowissenschaften, Universität Frankfurt, D-60438 Frankfurt, Germany; [email protected]
The correlation of radiometric mineral ages with particular P-T path segments may be hampered by the lack
of clear evidence at what stage a particular target mineral grew or (re-)equilibrated. Hence, the significance
of the ages obtained is often enough open to interpretation. Here, a geochronological study is presented in
which the minerals to be dated had been selected on the basis of petrological and microstructural criteria.
The samples are from the Mary Kathleen Fold Belt in the central Mount Isa Inlier, NE Australia, a classic
low-pressure, high-temperature metamorphic terrain characterised by a prograde P-T path passing through
the andalusite into the sillimanite stability field where peak P-T conditions had been reached at 550-650°C
(from lowest- to highest-grade zones) at about 4 kbar (Reinhardt, 1992). This prograde evolution was
initiated by a heating event and accompanied by substantial crustal shortening through folding (“D2”),
producing a pervasive axial planar foliation (“S2”). Syn-D2 minerals that are aligned within that foliation
include monazite and rutile. Thus, the U-Pb system in monazite was expected to yield an age for peak
metamorphism, both from microstructural constraints as well as from the temperatures reached. A large
number of monazites present in the schist matrix and as inclusions in andalusite and cordierite have been
analysed with the electron microprobe (method devised by Montel et al., 1996), which resulted in a unimodal
age peak with a mean value of 1581±5 Ma, based on nearly 400 analysis points. In-situ dating by LA-SFICP-MS of monazite grains in the schist matrix including S2-aligned monazite yielded a concordant U-Pb
age of 1580±4 Ma, which is practically identical to the microprobe result. Prograde rutile yielded U-Pb ages
of about 1570 Ma, most likely dating the cooling of the respective rocks to below 500°C, the closure
temperature for rutile.
Retrograde metamorphism is recorded in cordierite schists by incipient, and locally pervasive, breakdown of
cordierite due to H2O infiltration, resulting in aluminosilicate + chlorite + quartz assemblages. An
anticlockwise P-T evolution is well constrained by the successive formation of sillimanite, kyanite, and
andalusite, as observed in strongly re-hydrated rocks (Reinhardt 1992, 2011).
Retrograde overprinting produced random mineral orientations, which, in case of pervasive retrogression,
eliminated all signs of S2. Monazite and xenotime in strongly retrogressed samples show features that
suggest they have grown during hydration and are not inherited from the prograde assemblages. They tend to
form large, randomly oriented euhedral crystals, and monazite may show distinct concentric birefringence
zoning. Xenotime appears to be confined to retrograde samples. Compared to prograde monazite, LA-SFICP-MS dating of monazite and xenotime from two retrograde kyanite-chlorite-rich samples gave slightly
younger U-Pb ages of 1572±5 and 1573±4 Ma, respectively. Large retrograde rutile grains yielded U-Pb ages
identical to those of prograde rutile, again interpreted as marking the cooling stage at about 500°C.
The results from monazite, xenotime and rutile dating confirm that the anticlockwise P-T path is monocyclic.
In combination with thermobarometric constraints the age data indicate that the initial cooling from peak
temperatures to 500°C was relatively rapid, in the order of 10-15°C per million years. Hence, this regional
low-P-high-T orogenic event was of limited duration, compared with typical crustal shortening processes.
Reinhardt, J. (1992), Geol. Mag. 129, 41-57.
Reinhardt, J. (2011), Eur. J. Mineral. 23, 795-803.
Montel, J.-M., Foret, S, Veschambre, M., Nicollet, C., Provost, A. (1996), Chem. Geol. 131, 37-53.
67 ON THE ORIGIN OF ANKARAMITE
RJ Roberts
Department of Geology, University of Pretoria, Hatfield, Pretoria, South Africa; [email protected]
Ankaramites are defined as “a porphyritic melanocratic basanite with abundant phenocrysts of pyroxene and
olivine”. These rocks are a very minor but not uncommon constituent of many basaltic volcanic occurrences,
found on ocean islands (e.g. Marion, Hawaii, Gough), in island arcs (e.g. Vanuatu) and in flood basalts (e.g.
Powai in the Deccan Traps). Generally these rocks consist almost entirely of clinopyroxene, both as very
large phenocrysts and groundmass, with minor olivine contents. Considering the high Mg-nature of these
rocks, and the presence of olivine phenocrysts alongside and within the clinopyroxene phenocrysts, these
rocks are generally ascribed a mantle origin and treated as derivatives of mantle-derived magmas.
However, a closer inspection shows that these rocks may not be mantle-derived at all. Though the
clinopyroxenes feature a high Mg#, the clinopyroxenes are rich in Al and contain significant Fe3+, indicating
a higher oxygen fugacity in the melt or the incorporation of an oxidised assimilant. Similarly, the olivine
phenocrysts range from Fo75-Fo85, generally considered to be low for mantle-derived olivine. Adding in the
physical constraints of growing and then moving cm-scale phenocrysts any great distance, it has been argued
in two new papers from Haleakala (Hawaii) and Marion Island that ankaramites are not mantle-derived at all,
but come from shallow magma chambers undergoing the final stages of degassing.
This paper tackles the ankaramite problem globally, using data collated from a variety of sources to compare
and contrast reported “ankaramites”. This will lead to a reassessment and reclassification of some of these
rocks, and a greater understanding of their formation mechanisms.
68 PETROGENETIC RELATIONSHIPS BETWEEN THE KOENAP FORMATION MIGMATITES
AND THE SWARTOUP GRANODIORITE (CENTRAL NAMAQUA METAMORPHIC COMPLEX)
Graeme Schmeldt1 & Steffen H. Büttner1
1
Department of Geology, Rhodes University, Grahamstown, South Africa; [email protected];
[email protected]
The central Namaqua Metamorphic Complex shows widespread upper amphibolite and granulite facies
metamorphism over a long time period between at least ~1220 and <1100 Ma. Anatexis predominantly
affected felsic, but in places also mafic, supracrustal rocks, but the petrogenetic relationships between
migmatites and granites are poorly understood. In the Swartoup Hills, ~40 km east of Onseepkans, the
Swartoup Granodiorite and migmatites of the meta-pelitic/psammo-pelitic Koenap Formation show a close
spatial relationship to each other, and in places gradational boundaries may suggest the extraction of
anatectic magma to form the ca. 12 km long and 200-600 m thick layer of the Swartoup Granodiorite.
In a pilot study we have collected migmatites of various degrees of partial melting and melt depletion, and a
representative range of Swartoup plutonic rocks. On the basis of major and trace elemental compositions we
classify the different rock types and evaluate possible genetic relationships. As a first hypothesis we assume
that if the magma of the Swartoup plutons was formed by partial melting and melt extraction from the
Koenap Formation, the restitic anatexites should be depleted in those elements that are enriched in the
extracted melt (i.e. particularly the incompatible elements; LILE, HFSE). If the Swartoup pluton is
genetically related to partial melting and melt extraction in the Koenap migmatites, the undifferentiated
protolith composition should fall in-between the restitic Koenap migmatites and the segregated pluton
compositions. We assume that the Koenap Formation is of pelitic primary composition and therefore we
compare Koenap migmatites and the Swartoup granitoid with common average pelite compositions.
In the TAS diagram, Koenap migmatites plot in the granodiorite field with low alkali oxide contents (~2-5
wt%). The Swartoup pluton is also granodioritic but contains more alkali oxide (6 wt%). One sample plots in
the quartz-monzonite field with ~8 wt% Na+K oxide. The SiO2 contents uniformly range between 63 and 68
wt%. Unmetamorphic pelites tend to plot between the main clusters of the Koenap restites and the Swartoup
Granodiorite. Similar spatial relationships are seen in the R1/R2 diagram.
In the Swartoup Granodiorite, contents of incompatible elements, such as K and Rb, are similar to those in
common pelites, and significantly higher than in most Koenap migmatites. Also Zr and Sr are enriched in the
Swartoup Granodiorite (>400 ppm Zr, 150-340 ppm Sr) compared to 300-400 ppm Zr and 26-135 ppm Sr in
the migmatites. This may indicate high Zr solubility in the melt, and accordingly high melt temperatures.
Primary pelites are variably low in Zr (100-300 ppm), which may suggest that the Koenap protoliths may
have been either unusually rich in Zr or that a pre-anatectic process led to Zr enrichment. Similarly, Sr is
enriched in the Granodiorite (~150-340 ppm) compared to the migmatites (~26-135 ppm). All this may be in
agreement with melt extraction from the Koenap migmatites into the Swartoup Granodiorite.
Titanium, as a compatible element, is slightly higher in the restitic Koenap Formation compared to the
Swartoup Granodiorite, although biotite, probably an important Ti carrier in the pre-anatectic protolith,
largely broke down during partial melting. Possibly, peritectic ilmenite incorporated Ti released during
biotite breakdown and remained in the source. Phosphorous is enriched in the Swartoup Granodiorite and
depleted in the migmatites compared to common pelite compositions. The migmatites show a strong negative
Eu anomaly and, compared to the Swartoup Granodiorite, are enriched in HREE. The latter might correlate
with the abundance of garnet in the migmatites, a phase that is largely absent in the Swartoup Granodiorite.
Overall there appears to be some indication that the Swartoup Granodiorite may consist of magma extracted
from the Koenap migmatites, but further data evaluation, broadening of the sample base, and isotope analysis
is required to provide sufficient evidence on the actual genetic relationships.
69 MELT PRODUCTION AND BUFFERING IN THE NAMQUA METAMORPHIC COMPLEX,
SOUTH AFRICA
Simon Schorn & Johann Diener
Department of Geological Sciences, University of Cape Town, South Africa; [email protected]
The Namaqua Metamorphic Complex exposes mid- to lower-crustal igneous and high-grade metamorphic
rocks in a belt that extends for more than 1500 km across southern Africa. The studied locality at Hytkoras in
the granulite-facies part of the Bushmanland terrane of western Namaqualand hosts a variety of granulitefacies rocks as well as broadly granitic intrusives. The metamorphic rocks can be divided in three distinct
groups: (i) pelitic, aluminous granulites (g–bi–cd–sill–feld–q–melt), (ii) Mg–Fe–Al-rich ‘magnesian
gneisses’ (cd–bi–opx ± g ± feld ±q ± melt) and (iii) mafic granulites (aug–opx–pl–q–melt ± hb ± bi). PT
estimates from pseudosection modelling indicate peak conditions of 800–840 ºC and 5–6 kbar for all rock
types. Modelling employing representative melt-reintegrated bulk compositions suggest voluminous melt
production in all three lithologies corresponding to major melt-producing reactions. For the pelitic granulites
the suprasolidus muscovite-breakdown produces ~15 vol. % melt (plus peritectic ksp + sill) while the bi–sill
consuming melting reaction produces ~10 vol. % melt (plus abundant peritectic cd + g + ksp). For a
representative ‘magnesian gneiss’ the major melt-producing reaction corresponds to the g–bi consuming,
opx–cd producing reaction through which ~15 vol. % melt is produced. For the mafic granulites the major
melt-producing reaction corresponds to the terminal hornblende-breakdown in the opx stability field. The
latter reaction produces ~30 vol. % melt. In all investigated cases only minor melt is produced by continuous
melting prior to the onset of the named melting reactions. The fact that the inferred equilibrium assemblages
of the three rock types correspond to the named major melt-producing reactions is interpreted as symptom of
major heat consumption and -drainage. We suggest that – at constant heat input – the heat/energy consumed
during a major melting reaction is a first-order control on the dwelling time of an equilibrium assemblage in
specific fields of a pseudosection. This has potentially far-reaching implications on the interpretation of PTpaths and the PT-evolution of granulite terrains in general.
70 NEOARCHEAN THRUSTING OF GRANULITE FACIES METAPELITES OVER AMPHIBOLITE
FACIES GREY GNEISSES IN THE SOUTH MARGINAL ZONE OF THE LIMPOPO BELT:
IMPLICATIONS FOR INTERPRETING THE GEODYMANIC EVOLUTION OF THE
NORTHERN EDGE OF THE KAAPVAAL CRATON
G. Stevens, A Vezinet, JF Moyen, G Nicoli and D Frei
Centre for Crustal Petrology, Department of Earth Sciences, Stellenbosch University, South Africa.
[email protected]
Université de Lyon, Laboratoire Magmas et Volcans, UJM-UBP-CNRS-IRD, 23 rue Dr. 8 Paul Michelon,
42023 Saint Etienne, France.
Now at: Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK.
Now at: Department of Earth Science, University of the Western Cape, Bellville, South Africa.
The Southern Marginal Zone (SMZ) of the Limpopo Belt consists of the metapelitic, metamafic and metaultramafic units of the Bandelierkop Formation and the volumetrically dominant grey gneisses of the
Baviaanskloof gneiss unit. The migmatitic metapelites of the Bandelierkop Formation are characterised by
the development of cordierite-orthopyroxene symplectites after garnet. These metapelites have long been
understood to have undergone granulite facies anataxis by incongruent melting reactions that consumed
biotite, at temperatures in excess of 800 °C and pressures consistent with burial to in excess of 30km depth.
The good evidence for granulite facies metamorphism in these rocks was interpreted to indicate that the
entire SMZ had experienced equivalent conditions. Metamorphic and geochronological investigation of the
grey gneisses from a wide region of the SMZ indicates that this interpretation is incorrect. The metapelites
were deposited after 2733 ± 13 Ma, and accurately constrained peak metamorphic conditions of 852.5 ± 7.5
°C and 11.1 ± 1.3 kbar, were attained at 2713 ± 8 Ma. In contrast, some rocks that make up the
Baviaanskloof gneisses crystalized at approximately 3200 Ma, with the majority formed between 2950 and
2750 Ma. The metamorphic age recorded by the metapelites is not recorded by the rocks of the
Baviaanskloof gneiss unit. These gneisses commonly consist of the assemblages quartz + plagioclase +
biotite and quartz + plagioclase + biotite + hornblende + epidote. Phase equilibrium modelling of both
assemblages in relevant compositions indicates that they equilibrated under amphibolite facies conditions at
less than 8 kbar. Rare orthopyroxene-bearing grey gneisses are described in the literature, but these are
generally in close proximity to metapelitic units or to the igneous charnockites of the Matok complex.
In combination, these results argue strongly for the Bandelierkop formation metapelites and the
Baviaanskloof gneiss having experienced different metamorphic histories and that the metapelites were
thrust over the grey gneisses after 2713 ± 8 Ma, but before the intrusion of the Matok complex at 2686 ± 7
Ma. The thrust package may have included some grey gneisses that have experiences granulite facies, but
these are alothonous in the SMZ, as are the metasediments and possibly the rest of the Bandelierkop
Formation. The idea that the decompression recorded by the metapelites represents the uplift of the SMZ due
to thrusting of the SMZ over the northern margin of the Kaapvaal craton is no longer substantiated.
71 IDENTIFYING YOUNG HIMU-LIKE AND ANCIENT HIMU SOURCES IN INTRAPLATE
MAGMATISM
Q.H.A. van der Meer1,2, T.E. Waight1, J.M. Scott3 & C.Münker4
1
Department of Geosciences and Natural Resource Management (Geology Section), Copenhagen
University, Øster Voldgade 10, 1350 Copenhagen K, Denmark; [email protected]
2
Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
3
Department of Geology, University of Otago, Leith Street, Dunedin 9054, New Zealand
4
Institut für geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany
Continental intraplate magmas with isotopic affinities similar to HIMU are identified worldwide.
Involvement of an asthenospheric HIMU or HIMU-like source is contested because the characteristic
radiogenic Pb compositions at unradiogenic Sr and moderate Nd and Hf compositions can also result from
in-situ ingrowth in metasomatised lithospheric mantle. Sr-Nd-Pb-Hf isotopic composition of late Cretaceous
lamprophyre dikes from Westland, New Zealand provide new insight into the formation of a HIMU-like
alkaline intraplate magmatic province under the Zealandia microcontinent and former contiguous regions of
Gondwana. The oldest (102-100 Ma) calc-alkaline lamprophyres are compositionally similar to the
preceding arc-magmatism (206Pb/204Pb(i) = 18.6, 207Pb/204Pb(i) = 15.62, 208Pb/204Pb(i) = 38.6, 87Sr/86Sr(i) =
0.7063 – 0.7074, εNd(i) = -2.1 – +0.1 and εHf(i) = -0.2 – +2.3) and represent melts originating from
subduction-modified lithosphere. Alkaline dikes erupted on the inboard Gondwana margin shortly after
cessation of subduction (92-84 Ma) have heterogeneous isotopic properties: 206Pb/204Pb(i) = 18.7 to 19.4,
207
Pb/204Pb(i) = 15.60 to 15.65, 208Pb/204Pb(i) = 38.6 to 39.4, 87Sr/86Sr(i) = 0.7031 to 0.7068, εNd(i) = +4.5 to
+8.0 and εHf(i) = +5.1 to +8.0. Melt compositions point to an amphibole-bearing spinel facies lithosphere
source enriched by metasomatism that introduced, amongst many elements, U + Th and caused rapid
ingrowth to HIMU-like compositions. Importantly, we find no evidence for (fossil) plume or other ancient
enriched asthenospheric components and the HIMU-like source appears to have completely originated from
the complex local subduction history. A coeval episode of alkaline magmatism (mainly 98-82 Ma) occurred
outboard of Gondwana’s former active margin and on the Hikurangi Plateau (an oceanic large igneous
province accreted to and subducted beneath Zealandia in the Early Cretaceous) with compositions closer to
true HIMU (206Pb/204Pb(i) ≈ 20.5, 207Pb/204Pb(i) ≈ 15.7, 208Pb/204Pb(i) ≈ 40.0, εNd(i) ≈ 4.5 and εHf(i) ≈ 4.0). In
contrast to the inboard HIMU-like source, the radiogenic 207Pb/204Pb and unradiogenic Nd and Hf require an
ancient enriched source component. This magmatism is interpreted to represent melting of fossilised HIMU
OIB that resided under the Hikurangi oceanic plateau, and erupted through the plateau and the overlying
continental crust above subducted segments of the plateau. These genetically distinct but isotopically similar
intraplate reservoirs were separated by the down-going slab under Gondwana’s former active margin.
Magmatism of this HIMU type was locally replaced by the inboard HIMU-like type which became dominant
across Zealandia over time, producing melt whenever the lithosphere is destabilised.
72 GEOCHRONOLOGY AND HF ISOTOPES OF THE WITRIVIER AND DE KRAALEN
GREENSTONE BELTS, SOUTH EASTERN KAAPVAAL CRATON
V. van Schijndel1, G. Stevens1, D. Frei2, C. Lana3 & T. Zack4
1
Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa;
[email protected]
2
Department of Earth Sciences, University of the Western Cape, Private Bag X17, Bellville 7535, South
Africa
3
Departamento de Geologia (DEGEO), Universidade Federal de Ouro Preto, Minas Gerais 35400000, Brazil
4
Department of Earth Sciences, University of Gothenburg, 40530 Gothenburg, Sweden
The supracrustal sequences of the De Kraalen and Witrivier greenstone belts (DKGB and WGB) are
characterized by a predominance of mafic-ultramafic rocks intercalated with banded quartzites and calcsilicates. We report the first results of combined U-Pb and Lu-Hf isotope analyses of zircon and U-Pb on
rutile from these greenstone successions.
A layered amphibolite of the WGB has two predominant zircon populations at ca. 3.43 and 3.24 Ga, where
the older U-Pb ages are represented by xenocrystic cores and the younger ages by the rims. The Lu–Hf
isotope analyses for the cores yielded superchondritic εHft values at 1.43 ±1.8 and subchrondritic εHft values
at -2.98 ±1.9 for the rims. The latter lie within the same crustal evolution trend as the older cores indicating
that they had a similar source.
A calc-silicate sample from the DKGB gave a single U-Pb on zircon age of 3230 ±3 Ma. However, Cl
imaging shows that there are at least two texturally different zircon populations. This difference is not seen
in the U-Pb ages, but there are two distinct groups within in the Lu-Hf isotope data. One zircon population
consists of xenocrystic cores that have a very high Hf/Hf signature. These data plots above the Depleted
Mantle (DM) line. The other zircon population consists of small, CL-dark grains with CL-bright rims that
also have a juvenile Hf/Hf signature but these data plot under the DM-line. Possibly the unusual radiogenic
Hf cores could (i) have been affected by fluid infiltration during metamorphism or (ii) have a highly depleted
mantle source that was metasomatized by sediment-derived fluids that was isotopically more depleted than
the average depleted mantle.
Both greenstone successions record similar peak pressures at ca. 12-15 kbar, although De Kraalen
supracrustal rocks recorded peak temperatures between 600-650° C and the Witrivier supracrustal rocks
reached a peak temperature of ca. 800° C between ca. 3.22 to 3.20 Ga (Saha et al., 2010). Temperature
calculation based on Zr in rutile from a meta-quartzite of the DKGB gave a temperature estimate of 712° C.
This is however could be a minimum temperature since the meta-quartzite lacks zircon and may be under
saturated in Zr. U-Pb on rutile from the same sample gave a relative young age of 3088 ±13 Ma. We assume
this is a cooling or possibly a mixed age caused by the heat transfer during the intrusion of the Vrede granitegranodiorite. The latter is compositionally similar to the 3.1 Ga potassic Mpuluzi (Lochiel) granite intruding
the Barberton Granite-Greenstone Terrane and Ancient Gneiss Complex (e.g. Anhaeusser and Robb, 1983).
References:
Saha, L., Hofmann, A., Xie, H., Hegner, E., Wilson, A., Wan, Y., Liu, D. & Kröner, A., 2010. Zircon ages
and metamorphic evolution of the Archean Assegaai–De Kraalen Granitoid-Greenstone terrain, Southeastern
Kaapvaal Craton, American Journal of Science 310, 1384–1420.
Anhaeusser, C. R. & Robb, L. R., 1983, Chemical analyses of granitoid rocks from the Barberton Mountain
Land, in Anhaeusser, C. R., editor, Contributions to the Geology of the Barberton Mountain Land, National
Geodynamics Programme, Barberton Project: Special Publication of Geological Society of South Africa 9,
189–219.
73 TRACE METAL (Bi, Cd, Co) DISTRIBUTION IN SWARTBERG BASE METAL SULPHIDE ORES
B. P. von der Heyden1, C. L.-L. Ukena1 & M.-L. van Zyl1
1
Department of Earth Sciences, University of Stellenbosch, Stellenbosch, South Africa; [email protected]
The Swartberg ore deposit is one of four major Broken Hill-type deposits hosted in the Precambrian aged
meta-sediments of the Bushmanland Group, Northern Cape (South Africa). The Swartberg ore deposit
consists of two stacked and multiply-deformed ore bodies (i.e., the Upper Ore Body (UOB) and the Lower
Ore Body (LOB)) which are both mined for their polymetallic Zn-Pb-Cu-Ag sulphide ore mineral
assemblages. The presence of so-called “penalty elements”, however, can have a negative impact on the
economic viability of some parts of the ore deposit. These deleterious elements include bismuth (Bi),
cadmium (Cd) and cobalt (Co); and if their concentration exceeds 1000 ppm, the ore is subject to costly
penalties imposed by the client.
In this study, we investigate the distribution of Bi, Cd and Co among different sulphide mineral phases as a
function of host-rock lithology (and stratigraphic position), and as a function of spatial proximity to the hinge
zone of an isoclinal F2 fold (previously interpreted to be a hydrothermal vent zone (Stedman, 1980)). In both
the LOB and the UOB, sulphide mineralogy has a strong control on the distribution of deleterious elements.
Isomorphic substitution of Co for Fe in pyrite ensures that this mineral has the greatest capacity for hosting
Co (median Co concentration = ~1100 ppm). In the UOB, Cd is hosted predominantly in sphalerite (average
concentration: ~2000 ppm), whereas in the LOB, Cd was found in significant concentrations in both
sphalerite (up to 5324 ppm) and chalcopyrite (average concentration: 2188 ppm). Bismuth concentrates
predominantly in galena where it can attain concentration values of greater than 20 000 ppm in both the
UOB and the LOB.
Distinctive trends exist in the concentration of deleterious elements among the different sulphide minerals,
and these trends vary with distance from the F2 hinge zone and among the different rock types evaluated in
this study (e.g., quartzitic schist, massive magnetite, garnet quartzite etc.). The implications of these trends,
and of the sulphide mineral chemical variability, will be discussed in a context of ore genesis (and possible
remobilisation) and in a context of the implications that this understanding has for exploration strategies for
Broken Hill-type or SEDEX styles of mineralisation. Furthermore, the discussion will focus on the
anticipated effects that the cation substitution will have on the beneficiation strategies employed during the
ore processing of base metal sulphide ores.
Stedman, D.P. 1980. The structural geology and metamorphic petrology of Black Mountain, Namaqualand. Masters
dissertation. Johannesburg: University of the Witwatersrand
74 ORIGIN OF LOW δ18O VALUES IN THE CRETACEOUS KOEGEL FONTEIN COMPLEX:
EVIDENCE FOR PRE-EXISTING 18O-DEPLETION IN THE COUNTRY ROCK.
B.A. Whitehead, C. Harris, K. Mulder & C. Olianti
Department of Geological Sciences, University of Cape Town, South Africa; [email protected]
The ~135 Ma Koegel Fontein Complex is situated approximately 350 km north of Cape Town and is the only
igneous complex on the west coast of South Africa that appears to be related to rifting of Africa and South
America. Unlike similar aged complexes in Damaraland, the Koegel Fontein Complex has abnormally low
δ18O values in both the intrusive rocks and the immediate country rock gneiss. The majority of rocks
analysed have δ18O values < 6 ‰, with values as low as -5 ‰. These δ18O values are too low to result from
exchange between rocks and ambient meteoric water at the time of intrusion (Curtis et al., 2013). Instead, the
low δ18O values are thought to result from dehydration melting of, and the efflux of metamorphic fluid from,
pre-existing low-δ18O gneiss.
Figure 1. Simplified map of the Koegel Fontein Complex showing the δ18O value of samples analysed. The
Rietpoort Granite is the largest and youngest intrusion of the Koegel Fontein Complex. The complex also
comprises a large number of mafic-felsic dykes. The country rock is Namaqua aged metamorphic rock. The
low δ18O shear zone is located in the roof pendant of the Rietpoort Granite.
Initial 18O depletion may have occurred due to hydrothermal interaction between meteoric fluids and gneiss
along Proterozoic shear zones. A traverse across a NNW trending, moderately ESE dipping shear zone
reveals that the δ18O values drop from about +8 ‰ in the undeformed country rock to lower than -2 ‰ in the
ductilely deformed mylonite. The country rock is cut by a small number of vuggy quartz veins. Cross-cutting
relations and their development along brittle faults and joint sets indicate that these veins are Cretaceous in
age and formed during hydrothermal activity associated with development of the complex. Some of these
veins have δ18O values lower than -4 ‰ (mean = +0.30 ‰, max = +10.93 ‰, min = -4.81 ‰). They must
have formed from a low- δ18O fluid.
This study confirms that 18O depletion occurred via alteration by hydrothermal fluids of meteoric origin prior
to intrusion of the Koegel Fontein Complex. Low δ18O fluids and melts were subsequently produced during
the early stages of development of the Complex.
75 IN SITU Sr ISOTOPES AND TRACE ELEMENTS IN THE EASTERN BUSHVELD COMPLEX
A.H. Wilson1 & A. Zeh2
1
School of Geosciences, Wits University, South Africa; [email protected]
2
Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany.
In spite of its major economic significance and over half a century of research there remains uncertainty as to
the origin of the Bushveld Complex and the source of the magmas that it formed from. Recent studies are
now questioning even the basic tenet that it formed as a magma chamber at all. However, each part of
stratigraphic section of the Bushveld provides unique information as to how the section formed. The lower
Bushveld series made up of the Basal Ultramafic Sequence (BUS), the Marginal Zone and Lower Zone is of
particular importance because it allows detailed insight into the reservoirs that multiply contributed to the
formation and modification of the initial magmas. It also allows understanding of the development of the
first stages of the magma chamber.
Here we report on the first comprehensive study on intercumulus mineral assemblages, incompatible
trace elements and precise in situ Sr isotopes in plagioclase from over 130 core samples for this section
(Clapham compartment). The intercumulus mineral assemblage that crystallized from the trapped melt
comprises plagioclase, K-feldspar, biotite and quartz and a wide variety of accessory phases, including
zircon, loveringite and primary magmatic anhydrite, the first time the latter mineral has been reported in the
layered sequence of the Bushveld Complex. In theory, this assemblage, together with a proportion of the
cumulus phases, has the geochemical signature of the melt compositions from which the cumulates formed.
The concentrations of incompatible trace elements (ITE) reflect both the amount of trapped melt in the
cumulates as well as the composition of that melt. In this section ITE ratios and changes in initial 87Sr/86Sr at
2056 Ma (ranging from 0.7042 to 0.7076) give insight into the various mantle sources and the variety of
crustal contaminants. The most primitive values are associated with the most ultramafic rock units in the
BUS and the highest values are in the Marginal Zone norites considered to have formed from evolved
magma at the top of the early chamber prior to ingression of Lower Zone magma. However, systematic
changes in the ratio occur through the section pointing to mixing of different magma types and in different
proportions. Recently published studies of in situ Sr isotope in plagioclase in Critical Zone rocks reveal a
range in compositions suggestive of different sources for the cumulus plagioclase. In contrast, the cumulus
plagioclase in the norites of this study show only single populations within individual samples.
The evolved magma was affected by significant assimilation of sediments as evidenced by partly
digested metapelite xenoliths, particularly in the upper part of the Marginal Zone. Rare earth element (REE)
patterns combined with modelling of trace elements and initial 87Sr/86Sr confirm the strong crustal
contamination derived from both lower crust of the Kaapvaal Craton and the interacting sediments. The
Lower Zone shows a progressively increasing mantle signature upwards in the sequence with both flat and
steep LREE patterns indicating controls by primitive (but still crustally contaminated) and more highly
contaminated melts. However, the relatively low Mg# for orthopyroxene and olivine (maximum Mg# 0.88)
in the Lower Zone (contrasting with Mg# 0.92 in the BUS) requires a melt contribution of lower Mg# than
would be associated with a deep mantle source and at the same time having a LREE depleted composition
subsequently enriched by crustal material. Such a source may be indicative of enriched sub-continental
lithospheric mantle (E-SCLM) and a progressively greater melt component of an eclogitic protolith
contributing to the parental magmas. The source of heat for this massive, but short lived (less than 1 million
years in duration) melting event is likely to have been a mantle plume which gave rise to the initial and most
magnesian magmas but with subsequent melting of an eclogitic source. Both magmas were then subject to
crustal contamination from the lower crust and the enclosing sediments. The results of this study quantify the
roles of the various Bushveld reservoirs including the previously proposed metasomatized E-SCLM, most
likely the source of the PGE and Cr that must have been present.
76 UNUSUAL GEOLOGICAL FEATURES AND THEIR IMPLICATIONS
S.E. Zhang1,2 & J.E. Bourdeau1,2 & A.D. Fowler2 & I. Heureux3
1
Department of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;
[email protected]
2
Department of Earth Sciences, University of Ottawa, Ottawa, Canada;
3
Department of Physics, University of Ottawa, Ottawa, Canada;
Magma hybridization and miscibility are topics currently infrequently discussed in geological texts and
literatures. Magmas which are differentiating may hybridize poorly and the resultant mixture may exhibit
unusual textures such as emulsion textures which are difficult to interpret classically based on established
petrological and geological criteria. Chemical partitioning during the hybridization process implies chemical
disequilibrium between the participant melts. In addition to hybridization chemical partitioning,
crystallization results in further mass transport. The resultant microscopic chemical, textural and
macroscopic geological features may be perplexing to decipher, based on the assumption of homogeneous
initial magmas. Our work demonstrates that the Central Intrusion of the Isle of Rum is most likely a
candidate of intercumulus melt mingling and partial hybridization. The Central Intrusion is host to unusual
geological features such as the rays of mesh-like to branching plagioclase rays (plagioclase stellates).
Various textures and chemical data will be presented and discussed.
77